[llvm] [NFC] Move DroppedVariableStats to its own file and redesign it to be extensible. (PR #115563)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams commented:

There might be room for further code deduplication but the cost of the boilerplate might not be worth it, so I think this broadly LGTM, if @adrian-prantl is happy with this approach too. I've got some inline nits, mostly to do with comments and naming.

In future, please can you avoid squashing your changes when addressing reviewer feedback? It makes it more difficult to understand what has changed from the original patch (which means I find myself re-reviewing the whole thing rather than the changes). Especially with GitHub's tendency to swallow inline review comments. (It's fine to squish all your local commits between rounds of review ofc, and it's not a hard rule. There are times where it makes sense). 

https://github.com/llvm/llvm-project/pull/115563