Fix Issue 2834 - The GC will now call destructors on heap allocated structs

~~This PR is not quite ready to merge, as I need to add a set  of tests for it, and also need to get it to work on arrays of structures as well. I am creating it now to make sure that I've not gone and broken druntime terribly with my approach to fixing the issue, as well as to take feedback on my method of solving the issue.~~ And the link to the issue this solves is [here](https://issues.dlang.org/show_bug.cgi?id=2834).

In the case where a struct is a member of a class, the struct's destructor will be called after the body of the class's destructor.

This PR is now ready to be reviewed and hopefully merged. It is not feasible to implement calling destructors of structs in heap allocated arrays for various reasons, including the inability to know which elements of the array are actually initialized, as well as which elements are valid.

This requires D-Programming-Language/dmd#3727

https://issues.dlang.org/show_bug.cgi?id=2834
This was my idea, too.

But it would *require* the GC to be able to save the type info somewhere, which is not done by the current gc, CDGC or https://github.com/D-Programming-Language/druntime/pull/1022. (Here is a precise GC that does it: https://github.com/rainers/druntime/tree/gc_precise_nov14_ti).

ATM it is not an implementation detail of the GC. The callback from the GC just passes the currently available information (ptr, size, bits)