Adjust fallthrough comments to satisfy GCC7's -Wimplicit-fallthrough.

If we were using C++17, we could use [[fallthrough]] instead... but we are not.

c++/src/capnp/compiler/compiler.c++

@@ -520,8 +520,7 @@ kj::Maybe<Compiler::Node::Content&> Compiler::Node::getContent(Content::State mi
       }
 
       content.advanceState(Content::EXPANDED);
-      // no break
-    }
+    } // fallthrough
 
     case Content::EXPANDED: {
       if (minimumState <= Content::EXPANDED) break;
@@ -584,8 +583,7 @@ kj::Maybe<Compiler::Node::Content&> Compiler::Node::getContent(Content::State mi
       }));
 
       content.advanceState(Content::BOOTSTRAP);
-      // no break
-    }
+    } // fallthrough
 
     case Content::BOOTSTRAP: {
       if (minimumState <= Content::BOOTSTRAP) break;
@@ -597,8 +595,7 @@ kj::Maybe<Compiler::Node::Content&> Compiler::Node::getContent(Content::State mi
       content.sourceInfo = kj::mv(nodeSet.sourceInfo);
 
       content.advanceState(Content::FINISHED);
-      // no break
-    }
+    } // fallthrough
 
     case Content::FINISHED:
       break;

c++/src/capnp/compiler/node-translator.c++

@@ -997,7 +997,7 @@ bool NodeTranslator::BrandedDecl::compileAsType(
         addError(errorReporter,
             "As of Cap'n Proto 0.4, 'Object' has been renamed to 'AnyPointer'.  Sorry for the "
             "inconvenience, and thanks for being an early adopter.  :)");
-        // no break
+        // fallthrough
       case Declaration::BUILTIN_ANY_POINTER:
         target.initAnyPointer().initUnconstrained().setAnyKind();
         return true;
@@ -2741,8 +2741,7 @@ kj::Maybe<Orphan<DynamicValue>> ValueTranslator::compileValue(Expression::Reader
         return kj::mv(result);
       }
 
-      // No break -- value is positive, so we can just go on to the uint case below.
-    }
+    } // fallthrough -- value is positive, so we can just go on to the uint case below.
 
     case DynamicValue::UINT: {
       uint64_t maxValue = 0;

c++/src/capnp/dynamic.c++

@@ -740,6 +740,7 @@ DynamicValue::Builder DynamicStruct::Builder::init(StructSchema::Field field, ui
               (uint)type.which());
           break;
       }
+      KJ_UNREACHABLE;
     }
 
     case schema::Field::GROUP:

c++/src/kj/encoding.c++

@@ -728,6 +728,7 @@ int base64_encode_block(const char* plaintext_in, int length_in,
       result = (fragment & 0x0fc) >> 2;
       *codechar++ = base64_encode_value(result);
       result = (fragment & 0x003) << 4;
+      // fallthrough
   case step_B:
       if (plainchar == plaintextend) {
         state_in->result = result;
@@ -738,6 +739,7 @@ int base64_encode_block(const char* plaintext_in, int length_in,
       result |= (fragment & 0x0f0) >> 4;
       *codechar++ = base64_encode_value(result);
       result = (fragment & 0x00f) << 2;
+      // fallthrough
   case step_C:
       if (plainchar == plaintextend) {
         state_in->result = result;
@@ -907,6 +909,7 @@ int base64_decode_block(const char* code_in, const int length_in,
         ERROR_IF(fragment < -1);
       } while (fragment < 0);
       *plainchar    = (fragment & 0x03f) << 2;
+      // fallthrough
   case step_b:
       do {
         if (codechar == code_in+length_in) {
@@ -924,6 +927,7 @@ int base64_decode_block(const char* code_in, const int length_in,
       } while (fragment < 0);
       *plainchar++ |= (fragment & 0x030) >> 4;
       *plainchar    = (fragment & 0x00f) << 4;
+      // fallthrough
   case step_c:
       do {
         if (codechar == code_in+length_in) {
@@ -943,6 +947,7 @@ int base64_decode_block(const char* code_in, const int length_in,
       ERROR_IF(state_in->nPaddingBytesSeen > 0);
       *plainchar++ |= (fragment & 0x03c) >> 2;
       *plainchar    = (fragment & 0x003) << 6;
+      // fallthrough
   case step_d:
       do {
         if (codechar == code_in+length_in) {

c++/src/kj/filesystem-disk-unix.c++

@@ -936,6 +936,7 @@ public:
             // Retry, but make sure we don't try to create the parent again.
             return tryReplaceNode(path, mode - WriteMode::CREATE_PARENT, kj::mv(tryCreate));
           }
+          // fallthrough
         default:
           KJ_FAIL_SYSCALL("create(path)", error, path) { return false; }
       } else {

c++/src/kj/hash.c++

@@ -47,8 +47,10 @@ uint HashCoder::operator*(ArrayPtr<const byte> s) const {
   switch (len) {
   case 3:
     h ^= data[2] << 16;
+    // fallthrough
   case 2:
     h ^= data[1] << 8;
+    // fallthrough
   case 1:
     h ^= data[0];
     h *= m;