protoc-gen-go: generate deprecation comments for oneof fields
protoc-gen-go: generate deprecation comments for oneof fields

Change-Id: Ia16dd78f37407be7fef6127a751bfd95015b35e4
Cherry-Pick: 1862ec3428f6522d86aca1c21a0666ea5026bad7@5e0eda4b6dc0f7e3c4ed8b94d87f7f7186b20c9c
Original-Author: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-on: https://go-review.googlesource.com/c/151428
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

golang/protobuf,golang/protobuf