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

[via cfe-commits]

cor3ntin wrote:

I think we need more discussion before enabling that by default tbh.

as @HerrCai0907 mentioned people who do custom rtti or crtp sort of need to be able to use static_cast in some cases

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