[Bug 47710] objidl.h contains invalid C++

https://bugs.winehq.org/show_bug.cgi?id=47710

--- Comment #2 from Zebediah Figura  ---
(In reply to Zebediah Figura from comment #1)
> It seems MIDL generates a name for all anonymous UDTs. I'll send a patch.

Nope, I was wrong. It does generate a name for top-level declarations, which we
don't, but it doesn't generate a name for nested declarations. Instead the
right thing to do seems to be to add the structure type _STGMEDIUM_UNION.
Unfortunately Microsoft decided to violate the C spec (big surprise), so we
need to define NONAMELESSUNION in every source file that includes objidl.idl
before making this change.