[clang] [clang][AST][NFC] Add '[[fallthrough]]' to cases fall through (PR #85921)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> E.g:
> 
> https://github.com/llvm/llvm-project/blob/50801f1095d33e712c3a51fdeef82569bd09007f/clang/lib/Interpreter/IncrementalParser.cpp#L141-L151

I think this is a demonstration of exactly what we *don't* want to see in the code base. That's just purely noise IMO.

https://github.com/llvm/llvm-project/pull/85921