[ms-cxxabi] Set proper SRet flags for most functions; also handle empty struct arguments correctly

[Timur Iskhodzhanov]

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2013/3/21 Timur Iskhodzhanov <timurrrr at google.com>:
> Hi John,
>
> Please see the attached patch.
>
> It addresses most of the Clang-side change needed to fix http://llvm.org/PR13676
>
> The remaining things are:
Just in case, by "remaining" I meant "things I want to do as separate
changes a bit later".

> - Handle cdecl correctly for instance methods (they return structures
> differently from global cdecl functions, which is weird)
> - Push the LLVM-side of the change when I get feedback from Anton (see
> the other thread mentioning PR13676)
>
> Thanks,
> Timur