MS ABI: "Fix" passing non-POD structs by value to variadic functions

Of course, such code is horribly broken and will explode on impact.
That said, ATL does it, and we have to support them, at least a little
bit.

Fixes PR20191.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212508 91177308-0d34-0410-b5e6-96231b3b80d8