[llvm] [LLVM][Coverage][Unittest] Fix dangling reference in unittest (PR #147118)

[via llvm-commits]

https://github.com/jeremyd2019 approved this pull request.

LGTM, but I'd appreciate another review from someone else because I don't have a firm handle on what's going on here

https://github.com/llvm/llvm-project/pull/147118