-fsanitize=nullability should trigger on uninitialized pointers

https://godbolt.org/z/Mfn8f4vYb

```
#include
#include

int main() {
void* a = nullptr;

void* _Nonnull b;

void* _Nonnull c = nullptr; // Only this one is triggered

void* _Nonnull d = &c;
memset(&d, 0, sizeof(d));

fprintf(stderr, "Done");
return 0;
}
```

I can see that memset case can be hard, but uninitialized value with or without -ftrivial-auto-var-init=zero should be easy.