Add initial clang targeted compatible decls for Intrin.h. Step towards
a windows compatible builtin header.

Currently uses x86intrin.h for implementing intel intrinsics in a clang
specific manner.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189709 91177308-0d34-0410-b5e6-96231b3b80d8