[clang] [Clang] Diagnose down casting static_cast (PR #117914)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Generally LGTM but we're missing test coverage for the `; did you mean 'dynamic_cast'?` part of the diagnostic message. It'd be good to have a test with `-frtti` and `-fno-rtti` showing that we do the right thing there, including with the fix-it suggestion.

https://github.com/llvm/llvm-project/pull/117914