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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Sorry for the long wait on this review! A related issue cropped up regarding compile time performance of `-Wall` (https://github.com/llvm/llvm-project/issues/105959) and it seems to be caused by CFG creation.

I think we should run the test case from that issue against the changes in this patch (`-Wall` includes `-Wextra`) to see if it makes an even more noticeable change in compile times, just to be sure before we go ahead and land this. I was hopeful that CFG building got faster, but now I worry there may be edge cases that make CFG building particularly slow that we may need to investigate.

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