Add CI action that reports clang-tidy warnings as review comments.

[hzeller]

#xls-build-gardener

[hzeller]

@proppy I am trying to get this clang-tidy-pr-comments action to work. The 403 looks like the action does not have permission to write. I suspect you'd be the best person knowing how the permission system is set up and what token to use.

(Note, currently the action is https://github.com/hzeller/clang-tidy-pr-comments which is just a fork from the original https://github.com/platisd/clang-tidy-pr-comments for easier debugging).

[proppy]

@hzeller do you get the same error when running it on your fork? https://github.com/hzeller/clang-tidy-pr-comments

That would help us to rules out project specific configuration issues.