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

[Tobias Hieta via llvm-commits]

tru wrote:

Not sure why I can't reply directly to the comment about the filter:

We need the filter for libc++ at least, so the code could either just filter out libc++ file and accept everything else, but then we can't control what extensions we should format. I kind of like that it's explicit now.

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