[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)

[David Spickett via llvm-commits]

DavidSpickett wrote:

https://github.com/llvm/llvm-project/pull/80648 is a temporary fix if that helps.

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