[llvm] [workflows] Delete user branches that are too long (PR #82845)

[Michael Spencer via llvm-commits]

https://github.com/Bigcheese commented:

What behavior does this have when someone actually hits it? I assume it runs after the push, so would they just try to go create the PR and have it just not be present? Also if spr is programmatically creating PRs, it may create it before this runs, and I don't think you can delete a branch that is referenced by a PR.

https://github.com/llvm/llvm-project/pull/82845