[clang] [clang] Add -Wimplicit-fallthrough to -Wextra (PR #97926)

[Max Coplan via cfe-commits]

vegerot wrote:

@AaronBallman sure.  Should I still use http://llvm-compile-time-tracker.com/ , or do I run the benchmark another way? 

https://github.com/llvm/llvm-project/pull/97926