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

[Shubham Sandeep Rastogi via llvm-commits]

rastogishubham wrote:

While I didn't use any templates and I didn't think any lambdas were necessary either, I did make the code more reusable by factoring out the common code into the base class, does this work @OCHyams @adrian-prantl 

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