Add PRFCHW intrinsic support

- Add head 'prfchwintrin.h' to define '_m_prefetchw' which is mapped to
  LLVM/clang prefetch builtin
- Add option '-mprfchw' to enable PRFCHW feature and pre-define '__PRFCHW__'
  macro

llvm-svn: 178041
diff --git a/clang/lib/Headers/x86intrin.h b/clang/lib/Headers/x86intrin.h
index 68ce106..441b38a 100644
--- a/clang/lib/Headers/x86intrin.h
+++ b/clang/lib/Headers/x86intrin.h
@@ -46,6 +46,10 @@
 #include <popcntintrin.h>
 #endif
 
+#ifdef __PRFCHW__
+#include <prfchwintrin.h>
+#endif
+
 #ifdef __SSE4A__
 #include <ammintrin.h>
 #endif