[clang] fix bug that undefined internal is a warning only for -pedantic-errors (PR #98016)

[Constantin Kronbichler via cfe-commits]

ccrownhill wrote:

Oh that makes sense so

```
static void f();
void *k = _Generic(&f, void (*)(void) : 0, default : f);
```

should be ok since it is not in the result expression?

https://github.com/llvm/llvm-project/pull/98016