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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thanks for this! I think the direction is reasonable.

I'm a little bit worried about the behavior of the C++ Core Guideline check in clang-tidy though as it's now wholly redundant, isn't it? Perhaps that check should ensure this warning is enabled and otherwise do nothing? CC @HerrCai0907 @5chmidti @PiotrZSL 

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