[llvm] Add github workflow that checks if a private email address was used to contribute to the repo and warn in this case (PR #80514)

[Tobias Hieta via llvm-commits]

tru wrote:

I think we should tweak this action a bit. It probably shouldn't spam a comment on every change. We can just update or keep one comment at the time. Also I wonder if we should link or include some expanded info why we request people to change this setting and what it does for us. It's not clear for anyone who haven't been part of the conversations why we want it to be public. 

https://github.com/llvm/llvm-project/pull/80514