Mark nested builder as clean after clear is called

Omitting this step was leading to stale cached versions of nested messages.
See protocolbuffers#10624

Author: googleberg

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

@@ -199,6 +199,9 @@ public SingleFieldBuilderV3<MType, BType, IType> clear() {
       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;
   }