[clang] [Headers][X86] Enable constexpr handling for MMX/SSE/AVX/AVX512 PMADDWD/PMADDUBSW intrinsics (PR #161563)

[Simon Pilgrim via cfe-commits]

================
@@ -23,6 +23,9 @@
 
 #define __trunc64(x)                                                           \
   (__m64) __builtin_shufflevector((__v2di)(x), __extension__(__v2di){}, 0)
+#define __zext128(x)                                                           \
+  (__m128i) __builtin_shufflevector((__v2si)(x), __extension__(__v2si){}, 0,   \
----------------
RKSimon wrote:

don't forget to undefine this at the end of the file

https://github.com/llvm/llvm-project/pull/161563