[llvm] Add a pass to collect dropped variable statistics (PR #102233)

[J. Ryan Stinnett via llvm-commits]

jryans wrote:

Looks quite helpful, thanks for working on this! 😄 

Does this report variables that have been dropped entirely, or also ones which have lost partial coverage...? I can't quite tell from the code, as you do appear to track some variable line info...

https://github.com/llvm/llvm-project/pull/102233