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

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

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).

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