[llvm] [MergeFunctions] Fix thunks for non-instruction debug info (PR #82080)

[Shoaib Meenai via llvm-commits]

smeenai wrote:

The test failure is unrelated and caused by b3050f518cc57e064ab6b3f17fb00534bd5b85d6 (which was later reverted but my branch doesn't include the revert).

https://github.com/llvm/llvm-project/pull/82080