[llvm] [GitHub] Add greeting comment to opened PRs from new contributors (PR #72384)

[Tobias Hieta via llvm-commits]

tru wrote:

What's the difference between FIRST_TIME and FIRST_TIME_CONTRIBUTOR?

Also a question for @kbeyls - maybe we should note that committing to this repo is a sign that they have accepted the license and our "CLA". Not sure where that's documented right now, but I think it might be good to link to that so people have time to review it before the PR is accepted.

https://github.com/llvm/llvm-project/pull/72384