[llvm] Add DebugSSAUpdater class to track debug value liveness (PR #135349)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse commented:

This is looking good with various nits inline. I didn't look at the unit test as I ran out of time. Rhyme.

Various bits here are duplicated from SSAUpdater, which I believe is fine as trying to abstract it further would probably be a false economy. Some kind of comment is in order to signal to readers where it's duplicated from, IMO, not sure where or how to word it though. 

https://github.com/llvm/llvm-project/pull/135349