[Headers][X86] _mm_cmpgt_epi64 is only available on SSE42 targets (#160491)

smmintrin.h redefines __DEFAULT_FN_ATTRS half way through the file to
handle SSE42-only instructions - when we made _mm_cmpgt_epi64 constexpr
we failed to redefine __DEFAULT_FN_ATTRS_CONSTEXPR as well to match

Author: RKSimon

clang/lib/Headers/smmintrin.h

@@ -1534,9 +1534,16 @@ static __inline__ __m128i __DEFAULT_FN_ATTRS _mm_minpos_epu16(__m128i __V) {
    so we'll do the same.  */
 
 #undef __DEFAULT_FN_ATTRS
+#undef __DEFAULT_FN_ATTRS_CONSTEXPR
 #define __DEFAULT_FN_ATTRS                                                     \
   __attribute__((__always_inline__, __nodebug__, __target__("sse4.2")))
 
+#if defined(__cplusplus) && (__cplusplus >= 201103L)
+#define __DEFAULT_FN_ATTRS_CONSTEXPR __DEFAULT_FN_ATTRS constexpr
+#else
+#define __DEFAULT_FN_ATTRS_CONSTEXPR __DEFAULT_FN_ATTRS
+#endif
+
 /* These specify the type of data that we're comparing.  */
 #define _SIDD_UBYTE_OPS 0x00
 #define _SIDD_UWORD_OPS 0x01