[llvm] [DebugInfo] Use correct unit when creating variable across CU boundary (PR #133282)

[Jeremy Morse via llvm-commits]

jmorse wrote:

I got drawn away from this; last I recall was putting some time into finding a better test but without any success. If it's not a major issue, acceptable to land as-is?

https://github.com/llvm/llvm-project/pull/133282