Convert "missing syntax" warning to an actual warning

For some reason this warning was emitted as a log message rather than a
structured warning. Convert it to use the AddWarning API so that it gets
emitted with a file and line number by protoc, and is visible via the
error collection interface during programmatic use.

src/google/protobuf/compiler/parser.cc

@@ -643,10 +643,11 @@ bool Parser::Parse(io::Tokenizer* input, FileDescriptorProto* file) {
       // Store the syntax into the file.
       if (file != nullptr) file->set_syntax(syntax_identifier_);
     } else if (!stop_after_syntax_identifier_) {
-      GOOGLE_LOG(WARNING) << "No syntax specified for the proto file: " << file->name()
-                   << ". Please use 'syntax = \"proto2\";' "
-                   << "or 'syntax = \"proto3\";' to specify a syntax "
-                   << "version. (Defaulted to proto2 syntax.)";
+      AddWarning(
+        "No syntax specified. Please use 'syntax = \"proto2\";' or "
+        "'syntax = \"proto3\";' to specify a syntax version. "
+        "(Defaulted to proto2 syntax.)"
+      );
       syntax_identifier_ = "proto2";
     }
 

src/google/protobuf/compiler/parser_unittest.cc

@@ -223,7 +223,7 @@ TEST_F(ParserTest, WarnIfSyntaxIdentifierOmmitted) {
   FileDescriptorProto file;
   CaptureTestStderr();
   EXPECT_TRUE(parser_->Parse(input_.get(), &file));
-  EXPECT_TRUE(GetCapturedTestStderr().find("No syntax specified") !=
+  EXPECT_TRUE(error_collector_.warning_.find("No syntax specified") !=
               std::string::npos);
 }