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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Addressed the following so I don't forget them later:
* Only look at PRs to main.
* Do not comment if changes have been requested.

I will be back with a refactored version based on the discussion above.

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