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

[John McCall via cfe-commits]

rjmccall wrote:

Yeah, I don't think we need this change.  The comment is a little unnecessary, but it's not a big deal.  Really, the code ought to be using a visitor instead of a `switch`.

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