[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/FallingTub.java

@@ -0,0 +1,44 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import java.nio.*;
+import java.lang.*;
+import java.util.*;
+import com.google.flatbuffers.*;
+
+@SuppressWarnings("unused")
+public final class FallingTub extends Struct {
+  public void __init(int _i, ByteBuffer _bb) { __reset(_i, _bb); }
+  public FallingTub __assign(int _i, ByteBuffer _bb) { __init(_i, _bb); return this; }
+
+  public int weight() { return bb.getInt(bb_pos + 0); }
+  public void mutateWeight(int weight) { bb.putInt(bb_pos + 0, weight); }
+
+  public static int createFallingTub(FlatBufferBuilder builder, int weight) {
+    builder.prep(4, 4);
+    builder.putInt(weight);
+    return builder.offset();
+  }
+
+  public static final class Vector extends BaseVector {
+    public Vector __assign(int _vector, int _element_size, ByteBuffer _bb) { __reset(_vector, _element_size, _bb); return this; }
+
+    public FallingTub get(int j) { return get(new FallingTub(), j); }
+    public FallingTub get(FallingTub obj, int j) {  return obj.__assign(__element(j), bb); }
+  }
+  public FallingTubT unpack() {
+    FallingTubT _o = new FallingTubT();
+    unpackTo(_o);
+    return _o;
+  }
+  public void unpackTo(FallingTubT _o) {
+    int _oWeight = weight();
+    _o.setWeight(_oWeight);
+  }
+  public static int pack(FlatBufferBuilder builder, FallingTubT _o) {
+    if (_o == null) return 0;
+    return createFallingTub(
+      builder,
+      _o.getWeight());
+  }
+}
+