[clang] Split -Wcast-function-type into a separate group (PR #86131)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Oof, that one is a bit messier to resolve -- it's mixing unrelated reference and pointer types and hoping the casts correctly convert back and forth properly. The fix to that should probably be done separately from the sanitizer fix so we get reasonable code review from LLVM folks.

https://github.com/llvm/llvm-project/pull/86131