[llvm] [GitHub] Add workflows to manage merging of PRs from authors without commit access (PR #124910)

[David Spickett via llvm-commits]

DavidSpickett wrote:

The new version:
* Follows the less automated approach suggested by @tstellar .
* Waits for checks to finish before notifying, as @nikic requested.
* Fixes a bug where you could approve, then request changes, and the workflow would not check which one was most recent.

The PR description has been updated to explain the way it works.

I still have a feeling this might need more to it, but no use doing that without proof it would help anyone. Also I will almost certainly have to patch some part of this post-commit, so it's best to keep it small after all.

https://github.com/llvm/llvm-project/pull/124910