Add auto-generated header to C# generated files (#4038)

* Add auto-generated header to C# generated files

* regenerate C# protos

csharp/src/AddressBook/Addressbook.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: addressbook.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: addressbook.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf.Conformance/Conformance.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: conformance.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: conformance.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: map_unittest_proto3.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: map_unittest_proto3.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/test_messages_proto3.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/test_messages_proto3.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf.Test/TestProtos/UnittestCustomOptionsProto3.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: unittest_custom_options_proto3.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: unittest_custom_options_proto3.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportProto3.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: unittest_import_proto3.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: unittest_import_proto3.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublicProto3.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: unittest_import_public_proto3.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: unittest_import_public_proto3.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: unittest_issues.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: unittest_issues.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: unittest_proto3.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: unittest_proto3.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/unittest_well_known_types.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/unittest_well_known_types.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/Reflection/Descriptor.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/descriptor.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/descriptor.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/Any.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/any.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/any.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/Api.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/api.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/api.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/Duration.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/duration.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/duration.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/Empty.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/empty.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/empty.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/FieldMask.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/field_mask.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/field_mask.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/SourceContext.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/source_context.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/source_context.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/Struct.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/struct.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/struct.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/Timestamp.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/timestamp.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/timestamp.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/Type.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/type.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/type.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

csharp/src/Google.Protobuf/WellKnownTypes/Wrappers.cs

-// Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/wrappers.proto
+// <auto-generated>
+//     Generated by the protocol buffer compiler.  DO NOT EDIT!
+//     source: google/protobuf/wrappers.proto
+// </auto-generated>
 #pragma warning disable 1591, 0612, 3021
 #region Designer generated code
 

src/google/protobuf/compiler/csharp/csharp_reflection_class.cc

@@ -104,8 +104,10 @@ void ReflectionClassGenerator::Generate(io::Printer* printer) {
 
 void ReflectionClassGenerator::WriteIntroduction(io::Printer* printer) {
   printer->Print(
-    "// Generated by the protocol buffer compiler.  DO NOT EDIT!\n"
-    "// source: $file_name$\n"
+    "// <auto-generated>\n"
+    "//     Generated by the protocol buffer compiler.  DO NOT EDIT!\n"
+    "//     source: $file_name$\n"
+    "// </auto-generated>\n"
     "#pragma warning disable 1591, 0612, 3021\n"
     "#region Designer generated code\n"
     "\n"