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

[Louis Dionne via llvm-commits]

ldionne wrote:

One of the reasons why I'm mentioning this is that it would also be nice to have in mind the closing of stale PRs. We may not want to implement that right now, and we may never want to do that, but I'm thinking that with the proposed solution the notion of a stale PR becomes just a minor variation on the check you're already doing.

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