[ObjC] Properly annotate extensions for ARC.

Just like fields, some extension fieldnames can be named such that they appear
to have meaning to ARC. Add the annotation to the compiler will get things
correct.

Add a bunch of extensions to allow inspection on generation to ensure things
are correct.

src/google/protobuf/compiler/objectivec/objectivec_extension.cc

@@ -61,6 +61,11 @@ ExtensionGenerator::~ExtensionGenerator() {}
 void ExtensionGenerator::GenerateMembersHeader(io::Printer* printer) {
   std::map<string, string> vars;
   vars["method_name"] = method_name_;
+  if (IsRetainedName(method_name_)) {
+    vars["storage_attribute"] = " NS_RETURNS_NOT_RETAINED";
+  } else {
+    vars["storage_attribute"] = "";
+  }
   SourceLocation location;
   if (descriptor_->GetSourceLocation(&location)) {
     vars["comments"] = BuildCommentsString(location, true);
@@ -72,7 +77,7 @@ void ExtensionGenerator::GenerateMembersHeader(io::Printer* printer) {
   vars["deprecated_attribute"] = GetOptionalDeprecatedAttribute(descriptor_, descriptor_->file());
   printer->Print(vars,
                  "$comments$"
-                 "+ (GPBExtensionDescriptor *)$method_name$$deprecated_attribute$;\n");
+                 "+ (GPBExtensionDescriptor *)$method_name$$storage_attribute$$deprecated_attribute$;\n");
 }
 
 void ExtensionGenerator::GenerateStaticVariablesInitialization(