Fix StringRef build with newer libc++

Author: tsukinoko-kun

include/llvm/ADT/StringRef.h

@@ -573,7 +573,9 @@ namespace llvm {
 // HLSL Change Starts
 // StringRef provides an operator string; that trips up the std::pair noexcept specification,
 // which (a) enables the moves constructor (because conversion is allowed), but (b)
-// misclassifies the the construction as nothrow.
+// misclassifies the the construction as nothrow. Newer libc++ releases reject user
+// specializations of this trait outright, and also compute the trait correctly without help.
+#if !defined(_LIBCPP_VERSION)
 namespace std {
   template<>
   struct is_nothrow_constructible <std::string, llvm::StringRef>
@@ -588,6 +590,7 @@ namespace std {
     : std::false_type {
   };
 }
+#endif
 // HLSL Change Ends
 
 #endif