[clang] [Headers][X86] Allow MMX/SSE/AVX MOVMSK intrinsics to be used in constexpr (PR #161914)

[Simon Pilgrim via cfe-commits]

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@@ -207,7 +208,8 @@ let Features = "sse2", Attributes = [NoThrow, RequiredVectorWidth<128>] in {
   def maskmovdqu : X86Builtin<"void(_Vector<16, char>, _Vector<16, char>, char *)">;
 }
 
-let Features = "sse2", Attributes = [NoThrow, Const, RequiredVectorWidth<128>] in {
+let Features = "sse2",
+    Attributes = [NoThrow, Constexpr, RequiredVectorWidth<128>] in {
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RKSimon wrote:

This can be Const, Constexpr and merge with other sse2 intrinsics

https://github.com/llvm/llvm-project/pull/161914