Update tests in preparation for using the MS ABI for Win32 targets

In preparation for making the Win32 triple imply MS ABI mode,
make all tests pass in this mode, or make them use the Itanium
mode explicitly.

Differential Revision: http://llvm-reviews.chandlerc.com/D2401
Use the MS ABI for Win32 targets by default

In addition to being a sensible default, this is a huge improvement
in test coverage for the MS ABI: any bot that targets Win32 will
now run the test suite using the MS ABI by default.

Differential Revision: http://llvm-reviews.chandlerc.com/D2401