[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:

@boomanaiden154 Confirmed works on LLVM repo as well: https://github.com/llvm/llvm-project/pull/80627#issuecomment-1926177727

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