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

[via cfe-commits]

Sirraide wrote:

> 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.

Its seem pretty unnecessary, yeah. I was fine w/ the one in this pr mainly because of the comment; my thinking was that there ought to be a reason why this comment is there—maybe it’s not obvious that that was intended here; I’m not that familiar w/ codegen. 

But also, now that I think about it, a comment that states *that* there is fallthrough but not *why* is just stating the obvious... I suppose we could probably also do the opposite and look into removing `[[fallthrough]]` in cases like these?

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