MS syntax in Clang tests

[Renato Golin]

Folks,

We had a few buildbot failures already because of x86-specific syntax
in MS tests, which just assume all back-ends are built. This is not
true nor will be for the foreseeable future.

So please, if you're adding an x86-specific, use FileCheck constraints
like REQUIRES or XFAIL or XPASS or add it to a target specific
directory. Whatever you need to restrict it to a specific
architecture.

http://llvm.org/docs/TestingGuide.html#platform-specific-tests

cheers,
--renato