[llvm] [GitHub] Add Greeting comment to new PRs with useful information (PR #72249)

[Nikita Popov via llvm-commits]

nikic wrote:

I don't want this kind of comment posted on every single PR (our PR notifications are already very spammy as it is), but if were limited to `pull_request.author_association == FIRST_TIME_CONTRIBUTOR`, this may make sense.

https://github.com/llvm/llvm-project/pull/72249