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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Is this addressing a diagnostic you're seeing in the wild? This shouldn't be necessary because the `case` statements are adjacent to one another with only comments/whitespace between them: https://godbolt.org/z/oM7x65hq9

(I'd like to understand the motivation better because I don't think we want to add `[[fallthrough]]` to all adjacent cases in the code base, so this seems inconsistent.)

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