[llvm] [workflows] Close issues used for backports once the PR has been created (PR #80394)

[Nikita Popov via llvm-commits]

nikic wrote:

> > I think this makes sense. In the future we might want to avoid creating issues in the first place and e.g. accept the cherry-pick command inside PRs (so you can directly run it in the PR that fixed the issue).
> 
> I think you can already use the cherry-pick command inside of PRs, because PR comments also generate the issue_comment event.

An issue with that is that you would have to add the PR to the milestone first, even though it really shouldn't be part of it.

https://github.com/llvm/llvm-project/pull/80394