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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Tested on my fork, where I added the label `no-commit-access`. We can bikeshed the name, I went with that because I wanted something that did not imply the PR is ready to merge, like "merge-on-behalf" might.

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