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

[Timur Iskhodzhanov]

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:
- 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
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