Clean up variable template handling a bit, and correct the behavior of name
lookup when declaring a variable template specialization.
Add implementations of _cpuid and _xgetbv to Intrin.h

The _cpuid() implementation is the same as in lib/Headers/cpuid.h
with the parameter names adjusted to match the interface.

_xgetbv just does what the Intel manual says.

Differential Revision: http://llvm-reviews.chandlerc.com/D2564