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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman please review
> 
> I don't know how to go about measuring if there even is a performance regression. How would I do that?

I would put up a branch on http://llvm-compile-time-tracker.com (instructions in the `About` page) and see how this impacts compile times. We've always left diagnostics out of -Wextra that require a CFG, so getting performance measurements is important.

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