Remove _mm_prefetch macro definition and associated _MSC_VER ifndefs

Author: rnk

clang/lib/Headers/xmmintrin.h

@@ -2197,10 +2197,6 @@ _mm_storer_ps(float *__p, __m128 __a)
 #define _MM_HINT_T2  1
 #define _MM_HINT_NTA 0
 
-#ifndef _MSC_VER
-/* FIXME: We have to #define this because "sel" must be a constant integer, and
-   Sema doesn't do any form of constant propagation yet. */
-
 /// Loads one cache line of data from the specified address to a location
 ///    closer to the processor.
 ///
@@ -2225,9 +2221,10 @@ _mm_storer_ps(float *__p, __m128 __a)
 ///    be generated. \n
 ///    _MM_HINT_T2: Move data using the T2 hint. The PREFETCHT2 instruction will
 ///    be generated.
-#define _mm_prefetch(a, sel) (__builtin_prefetch((const void *)(a), \
-                                                 ((sel) >> 2) & 1, (sel) & 0x3))
-#endif
+///
+/// _mm_prefetch is implemented as a "library builtin" directly in Clang,
+/// similar to how it is done in MSVC. Clang will warn if the user doesn't
+/// include xmmintrin.h or immintrin.h.
 
 /// Stores a 64-bit integer in the specified aligned memory location. To
 ///    minimize caching, the data is flagged as non-temporal (unlikely to be