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

[David Spickett via llvm-commits]

DavidSpickett wrote:

I realised a few things.

1. This should only run on PRs targeting the `main` branch, I will fix that.

2. Maybe some areas are using [required reviewers](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches#require-pull-request-reviews-before-merging). I assume the release branches are but that's out of scope for this.

libcxx [documents](https://libcxx.llvm.org/Contributing.html#the-review-process) that you should have 2 approvals from the reviewers group, but does not seem to use any GitHub feature to enforce that. Is that the case @ldionne ?

Related to that, it should be easy for use to opt out any given sub-project from this workflow by ignoring PRs with certain labels. So there is a quick workaround if it does mess up an existing review process.

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