[DDMD] Issue 13161 - Wrong offset of extern(C++) class member

https://issues.dlang.org/show_bug.cgi?id=13161
Walter, your compatibility things don't work.  We break abi compatibility all the time.  You are wasting your time and adding cruft to the source.

> Even this PR builds in a gratuitous difference between C++ and D alignment. Why?

Actually, it removes a gratuitous difference between C++ and D alignment on all platforms except windows.  It will also reduce memory consumption in some cases, as you can see from the modified test of classinfo size.