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
Sure, I just thought that using ```|=``` would be less order dependent (i.e. for future changes,) and that the optimizer could figure out that OR'ing with zero is the same as assignment.