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
Thanks for the info. We'll take out the assertion then. __setArrayAllocLength already checks whether the size is too large for the memory block, so we could assert on the return code of __setArrayAllocLength instead.