[llvm] [LiveDebugVariables] Add basic verification (PR #68703)

[Jeremy Morse via llvm-commits]

jmorse wrote:

In principle this LGTM, although it's been a long time since I've looked at LiveDebugVariables. As far as I recall, we use `setUnchecked` to do manual maintenance of ranges but they should all coalesce in the end. And it doesn't make sense for there to be a zero-length range of variable location.

This does involve iterating over every range of every variable in the function, which might have compile-time performance implications. Is it worth putting this behind an EXPENSIVE_CHECKS ifdef? (I haven't followed the SlotIndex related problems you've mentioned, I don't know whether it needs running on every compilation) 

https://github.com/llvm/llvm-project/pull/68703