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

[Nathan Chancellor via cfe-commits]

nathanchance wrote:

This seems reasonable to me. From the perspective of the Linux kernel, this seems like it should be a no-op, as we enable `-Wcast-function-type` and disable `-Wcast-function-type-strict` (under a normal build, it is on with `W=1`) explicitly:

```
$ rg 'cast-function-type(-strict)?\)' scripts/Makefile.extrawarn
57:KBUILD_CFLAGS += $(call cc-option, -Wcast-function-type)
134:KBUILD_CFLAGS += $(call cc-disable-warning, cast-function-type-strict)
```

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