[C++] Fix compile failure on Object API union construction for struct member (#6923)

* Add dedicated traits to Object API version of unions.

* Add suppression for unused parameters on unions of structs.

tests/union_vector/GadgetUnion.java

@@ -0,0 +1,33 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import com.google.flatbuffers.FlatBufferBuilder;
+
+public class GadgetUnion {
+  private byte type;
+  private Object value;
+
+  public byte getType() { return type; }
+
+  public void setType(byte type) { this.type = type; }
+
+  public Object getValue() { return value; }
+
+  public void setValue(Object value) { this.value = value; }
+
+  public GadgetUnion() {
+    this.type = Gadget.NONE;
+    this.value = null;
+  }
+
+  public FallingTubT asFallingTub() { return (FallingTubT) value; }
+  public HandFanT asHandFan() { return (HandFanT) value; }
+
+  public static int pack(FlatBufferBuilder builder, GadgetUnion _o) {
+    switch (_o.type) {
+      case Gadget.FallingTub: return FallingTub.pack(builder, _o.asFallingTub());
+      case Gadget.HandFan: return HandFan.pack(builder, _o.asHandFan());
+      default: return 0;
+    }
+  }
+}
+