Apply std::launder() where necessary

class B { ~~~~ };
class D1 : public B { ~~~~ };
class D2 : public B { ~~~~ };
static_assert(sizeof(D1) == sizeof(D2));

void mutate(B *b) // out-of-line, so compiler doesn't see what's going on
{
    assert(dynamic_cast<D1*>(b) || dynamic_cast<D2*>(b));
    b->~B();
    new (b) D2();
}

D1 d;
B *b = &d;
b->someVirtualCall();
mutate(b);
b->someVirtualCall(); // compiler may assume vtbl didn't change and call D1::someVirtualCall() directly
std::launder(b)->someVirtualCall(); // OK, compiler musn't de-virtualize