Constant 0p (NULL) does not work at the global declaration level

[pabuhr]

int & q = *0p; // does not work
int main() {
    int  & q = *0p; // works
}

The reason the global usage fails is that 0p is a function call, but at the global level, this call must be wrapped into a constructor and it is not wrapped.

Other global function calls are wrapped correctly.

[ajbeach]

Duplicated in #171, which contains some other (possibly useful) information.

[Thierry Delisle]

Fixed but not with proper constexpr. I just hacked it in in codegen.