[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/HandFanT.java

@@ -0,0 +1,20 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import java.nio.*;
+import java.lang.*;
+import java.util.*;
+import com.google.flatbuffers.*;
+
+public class HandFanT {
+  private int length;
+
+  public int getLength() { return length; }
+
+  public void setLength(int length) { this.length = length; }
+
+
+  public HandFanT() {
+    this.length = 0;
+  }
+}
+