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

[Max Coplan via cfe-commits]

vegerot wrote:

> CTMark is not compiled with `-Wextra`, so we'd not get any useful data out of this patch. What changes does one have to do to enable `-Wimplicit-fallthrough` by default (not just for `-Wextra`)?

@nikic done!  At [vegerot/llvm-project/perf/add-implicit-fallthrough-to-wextra](https://github.com/vegerot/llvm-project/tree/perf/add-implicit-fallthrough-to-wextra)

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