Suppress useless -Wshadow warning when using _mm* macros from emmintrin.h

Fixes <rdar://problem/10679282>.

I'm not completely satisfied with this patch.  Sprinkling "diagnostic ignored"
_Pragmas throughout this file is gross, but I couldn't suppress
it for the entire file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192143 91177308-0d34-0410-b5e6-96231b3b80d8