[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/falling-tub.ts

@@ -0,0 +1,63 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import * as flatbuffers from 'flatbuffers';
+
+
+
+export class FallingTub {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+__init(i:number, bb:flatbuffers.ByteBuffer):FallingTub {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+weight():number {
+  return this.bb!.readInt32(this.bb_pos);
+}
+
+mutate_weight(value:number):boolean {
+  this.bb!.writeInt32(this.bb_pos + 0, value);
+  return true;
+}
+
+static getFullyQualifiedName():string {
+  return 'FallingTub';
+}
+
+static sizeOf():number {
+  return 4;
+}
+
+static createFallingTub(builder:flatbuffers.Builder, weight: number):flatbuffers.Offset {
+  builder.prep(4, 4);
+  builder.writeInt32(weight);
+  return builder.offset();
+}
+
+
+unpack(): FallingTubT {
+  return new FallingTubT(
+    this.weight()
+  );
+}
+
+
+unpackTo(_o: FallingTubT): void {
+  _o.weight = this.weight();
+}
+}
+
+export class FallingTubT {
+constructor(
+  public weight: number = 0
+){}
+
+
+pack(builder:flatbuffers.Builder): flatbuffers.Offset {
+  return FallingTub.createFallingTub(builder,
+    this.weight
+  );
+}
+}