This mainly affects 64-bit winelib applications, and potentially mingw-w64
usage of the Wine headers. As explained in commit
fabfa59a, MSVC returns aggregates through an
implicit parameter immediately after the this/interface pointer. GCC's
"ms_abi" attribute is supposed to match this, but unfortunately current
versions of g++ (confirmed up to at least g++ 6.3) place the implicit return
pointer before the this/interface pointer. See also
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52792.

This commit takes an approach similar to the earlier commit
fabfa59a: For ABI compatibility the return
pointer is listed as an explicit parameter in the vtbl entry, while an inline
helper is provided for source code compatibility.
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>