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

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lb90 wrote:

> That said, if a patch went to Flex to convert them to some form of the add a FALLTHROUGH macro defined to the right thing, I think it makes objections a nothingburger.

Nice idea, opened https://github.com/westes/flex/pull/709. Let's see how it goes

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