Merge pull request #84606 from swiftlang/rebranch

Merge clang 21.x rebranch into main

lib/Serialization/Serialization.cpp

@@ -6358,6 +6358,9 @@ public:
   void writeTypeCoupledDeclRefInfo(clang::TypeCoupledDeclRefInfo info) {
     llvm_unreachable("TypeCoupledDeclRefInfo shouldn't be reached from swift");
   }
+  void writeHLSLSpirvOperand(clang::SpirvOperand) {
+    llvm_unreachable("SpirvOperand shouldn't be reached from swift");
+  }
 };
 
 }