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

[Tom Stellard via llvm-commits]

tstellar wrote:

> > For git push at least, deleting the branch closes the PR.
> 
> Do actions have the ability to make comments on PRs? It would be less user-hostile to have it leave a comment explaining what happened, and why their PR was being closed.

Yes, we can do that.  Can we always guarantee there will be a PR?  If not, I was thinking we could create an issue and @mention the user.

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