[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)

[Anton Korobeynikov via llvm-commits]

asl wrote:

I've added link to Discourse in https://github.com/llvm/llvm-project/commit/ee06678a7500d5d8f6aa8d2442389cdb90417c38

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