[llvm] Add a pass to collect dropped var statistics for MIR (PR #126686)

[Shubham Sandeep Rastogi via llvm-commits]

rastogishubham wrote:

@nikic I measured the time to compile clang with and without this patch using `time ninja clang` and noticed no significant difference in compile time. Please let me know if this is unsatisfactory in any way.

Without patch:
ninja clang  5289.37s user 449.03s system 1252% cpu 7:38.06 total


With Patc:
ninja clang  5326.41s user 446.89s system 1270% cpu 7:34.38 total

https://github.com/llvm/llvm-project/pull/126686