[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 already checked this in our temp-issue-tester repo and it seems to be working. We might miss some cases, but we can examine the things afterwards to see what they are.

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