[clang][ExprConst] Consider integer pointers of value 0 nullptr (#150164)

When casting a 0 to a pointer type, the IsNullPtr flag was always set to
false, leading to weird results like a pointer with value 0 that isn't a
null pointer.

This caused

```c++
struct B { const int *p;};
template<B> void f() {}
template void f<B{nullptr}>();
template void f<B{fold(reinterpret_cast<int*>(0))}>();
```

to be valid code, since nullptr and (int*)0 aren't equal. This seems
weird and GCC doesn't behave like this.

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