Move storage of final vtbl from BaseClass to IrInterface: BaseClass instances are shared! Always use the target's irstruct interfaceMap when building the const initializer. Tango helloworld works again.

module structs7;

struct S
{
    int i;
    long l;
}

void main()
{
    S s = void;
    int i = s.i;
    long l = s.l;
}