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

[Constantin Kronbichler via cfe-commits]

ccrownhill wrote:

Sorry, my mistake. This doesn't work for `default` but the problem is that in this example

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

the `undefined-internal` error is still raised even though it is not in the result expression anymore.

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