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

[Nikita Popov via cfe-commits]

nikic 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`)?

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