clang-cl requires fully qualified name for friend class declarations

src/google/protobuf/map_field.h

@@ -156,7 +156,7 @@ class PROTOBUF_EXPORT MapFieldBase {
   friend class ContendedMapCleanTest;
   friend class GeneratedMessageReflection;
   friend class MapFieldAccessor;
-  friend class DynamicMessage;
+  friend class ::PROTOBUF_NAMESPACE_ID::DynamicMessage;
 
   // Virtual helper methods for MapIterator. MapIterator doesn't have the
   // type helper for key and value. Call these help methods to deal with