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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

If possible, it'd be better to have them in the same commit with a ["Co-authored-by" tag](https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors); it's not the worst thing if the fix lands first and then the test, but if the functional commit causes some further error and gets reverted after the unit test has landed, then the unit test will still be present and will start failing because of the revert. It's not a common scenario, but using a single commit reduces the risk of confusion or surprising behaviour.

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