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

[Tom Stellard via llvm-commits]

tstellar 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.

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