[llvm] [Workflow] Add new code format helper. (PR #66684)

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

I really like this, thanks for setting that up! I am a tiny bit concerned about the comments being extra verbose, but I guess we can start that way and adjust as needed.

I'd like @mordante and @philnik777 to take a look in case I'd have missed something but this LGTM for libc++.

A notable behavior change is that we'll now start enforcing that any modified file in `libcxx/test` is `clang-formatted`, but I think that's OK. We could handle that by making our `ignore_format.txt` file contain more entries if needed.

https://github.com/llvm/llvm-project/pull/66684