[clang] [Clang] Add -Wimplicit-fallthrough to -Wextra for GCC compatibility (PR #149474)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

One concern I have is about compile time performance impacts; the implicit fallthrough diagnostics require building a CFG and that can be expensive. It may be worth putting up a branch on https://llvm-compile-time-tracker.com/ to see how it impacts compile times

https://github.com/llvm/llvm-project/pull/149474