[Bug 54226] New: widl: implement `cstruct_out` option to generate C bindings compatible with small struct return values

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=54226

Bug ID: 54226
           Summary: widl: implement `cstruct_out` option to generate C
                    bindings compatible with small struct return values
           Product: Wine
           Version: unspecified
          Hardware: x86-64
                OS: Windows
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tools
          Assignee: [e-mail]
          Reporter: [e-mail]

According to https://devblogs.microsoft.com/oldnewthing/20220113-00/?p=106152,
the previous C bindings are incompatible with the C++ COM ABI when the method
returns a small struct for MSVC. Their solution is to add the `/cstruct_out`
flag to the MIDL compiler
(https://learn.microsoft.com/en-us/windows/win32/midl/-cstruct-out, added in
the Win11 SDK according to comments) to generate new C bindings compatible with
the C++ COM ABI.

This is similar to the (long-standing) GCC issue with its mingw-w64 MS ABI
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64384), which has been worked
around in widl/mingw-w64 by having special wrapper functions in the C++
bindings, guarded by `WIDL_EXPLICIT_AGGREGATE_RETURNS`.

I think widl should follow suit to implement a `--cstruct_out` flag to do the
same thing for the C bindings. This does not need to be guarded by
`WIDL_EXPLICIT_AGGREGATE_RETURNS` because both GCC and MSVC has the same issue
and will require the same fix. (I do not know if we need some kind of guard for
the mingw-w64 headers to revert to the old function declarations, but given
that the old bindings are practically broken perhaps it doesn't matter.)

(I have not checked what `/cstruct_out` on MIDL actually does to the C
bindings.)