Add ClassInfo flag for destructors and don't finalize dtor-less objects.

DMD pull: https://github.com/D-Programming-Language/dmd/pull/3725
Inspired by: https://github.com/D-Programming-Language/druntime/pull/872
Even if a C++ class has a dtor, it doesn't have a classinfo pointer in it's vtable so the gc won't be able to call it - it will crash if the finalize bit is set.