[DDMD] Issue 13707 - msvc32 C++ struct return ABI not followed for structs with constructors

A constructor should stop a struct from being passed in registers on win32/C++.

https://issues.dlang.org/show_bug.cgi?id=13707
> Doesn't that also apply to LINKc?

Yes and no.  The corresponding C declaration of course doesn't have a constructor, but an `extern "C"` C++ function does...  I'm not really sure which way to go on this, so sticking with the current behavior seems safe.