cleanup: Make .clang-tidy consistent. (googleapis/google-cloud-cpp-spanner#154)
cleanup: Make .clang-tidy consistent. (googleapis/google-cloud-cpp-spanner#154)

I would like us to share these files, but that sounds hard, we should
keep them consistent in any case.

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp