[Runtime] Copy the value witness table pointer manually.

Fixes rdar://74136916.

Author: varungandhi-apple

stdlib/public/runtime/Metadata.cpp

@@ -392,6 +392,10 @@ static void swift_objc_classCopyFixupHandler(Class oldClass, Class newClass) {
   if (!oldClassMetadata->isTypeMetadata())
    return;
 
+  // Copy the value witness table pointer for pointer authentication.
+  auto newClassMetadata = reinterpret_cast<ClassMetadata *>(newClass);
+  newClassMetadata->setValueWitnesses(oldClassMetadata->getValueWitnesses());
+
  // Otherwise, re-sign v-table entries using the extra discriminators stored
  // in the v-table descriptor.