[llvm] [GitHub] Add workflow to check author's commit access on new PRs (PR #123593)

[David Spickett via llvm-commits]

DavidSpickett wrote:

I will leave this up for a few days because A: there's no rush and B: it has been suggested to have a separate discussion about reviewers merging on behalf, and that might result in a different solution to this.

https://github.com/llvm/llvm-project/pull/123593