[clang] [clang] move -Wcast-function-type under -Wextra (PR #77178)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Yeah, I was hoping to avoid having to introduce another diagnostic group, but I think that's going to be the best way forward. I've got a patch in progress to add `-Wcast-function-type-mismatch` so we end up with:
```
-Wcast-function-type
|- -Wcast-function-type-mismatch
|- -Wcast-function-type-strict
```
and then add `-Wcast-function-type-mismatch` to `-Wextra`, but still thinking through the changes and whether there's a different approach.

https://github.com/llvm/llvm-project/pull/77178