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

[Max Coplan via cfe-commits]

vegerot wrote:

> 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.

Thanks!  The `About` page says

> If you are an LLVM contributor who **regularly** does compile-time sensitive work

Do you think I should still ask for permission or should I leave it to someone else?

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