message: Preserve protobuf comments in generated go code
  bazelbuild/rules_go#2646 has some additional information on this
  Risk Level: Low
  Fixes envoyproxy/go-control-plane#233
diff: @@ -28,7 +28,11 @@ def generateProtobufs(output):
  # Each rule has the form @envoy_api//foo/bar:baz_go_proto.
  # First build all the rules to ensure we have the output files.
  - check_call(['bazel', 'build', '-c', 'fastbuild'] + go_protos)
  + # We preserve source info so comments are retained on generated code.
  + check_call([
  + 'bazel', 'build', '-c', 'fastbuild',
  + '--experimental_proto_descriptor_sets_include_source_info'
  + ] + go_protos)
  for rule in go_protos:
  # Example rule:
  