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

[Erich Keane via cfe-commits]

erichkeane wrote:

> If we don't feel like enabling this by default, a strategy would be to attach this warning to some `-fhardened` flag (as available in GCC)

IMO, this is useful as on-by-default, but under its own flag.  I haven't looked at the 'diff' yet, but that is my thoughts on it.

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