[llvm] [GitHub][workflows] Ask reviewers to merge PRs when author cannot (PR #81142)

[David Spickett via llvm-commits]

DavidSpickett wrote:

I modified the comment for when the reviewer also lacks permissions, what do you think @tstellar ?

Discord seems the obvious place to find people online at the time, or they can @ another reviewer to have them do it.

https://github.com/llvm/llvm-project/pull/81142