[llvm] Add unit test for CloneModule and #dbg_label (PR #91463)

[Harald van Dijk via llvm-commits]

hvdijk wrote:

If it had been my own test, I would have included it in my earlier PR, but it's Pietro's test, so I think it's fairest to have him listed as the author, and Git makes that easiest by having separate PRs. But if you prefer, we can merge them.

https://github.com/llvm/llvm-project/pull/91463