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

[David Spickett via llvm-commits]

DavidSpickett wrote:

> Maybe it would be better to include this workflow as the first job as the labeler (.github/workflows/new-prs.yml) and have the labeler depend on that job having run (or been skipped). Then we would always have greeting (or not) - labelling - subscribers.

I will work on this today so you all can at least see what it would look like. If it's relatively neat I think the guaranteed order would be nice to have.

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