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

[Peter Dimov via cfe-commits]

pdimov wrote:

Since you aren't diagnosing reinterpret-casting a function pointer from `void*`, because this would warn on idiomatic POSIX code (`dlsym` returns `void*`), it seemed to me that you'd be interested in not warning on idiomatic Windows code (`GetProcAddress` returns `FARPROC`.)

But perhaps I was mistaken.

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