Mark nested builder as clean after clear is called

Omitting this step was leading to stale cached versions of nested messages.
See https://github.com/protocolbuffers/protobuf/issues/10624

java/core/src/main/java/com/google/protobuf/SingleFieldBuilderV3.java

@@ -199,6 +199,9 @@ public class SingleFieldBuilderV3<
       builder = null;
     }
     onChanged();
+    // After clearing, parent is dirty, but this field builder is now clean and any changes should
+    // trickle up.
+    isClean = true;
     return this;
   }