[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/hand-fan.ts

@@ -0,0 +1,87 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import * as flatbuffers from 'flatbuffers';
+
+
+
+export class HandFan {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+__init(i:number, bb:flatbuffers.ByteBuffer):HandFan {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+static getRootAsHandFan(bb:flatbuffers.ByteBuffer, obj?:HandFan):HandFan {
+  return (obj || new HandFan()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+static getSizePrefixedRootAsHandFan(bb:flatbuffers.ByteBuffer, obj?:HandFan):HandFan {
+  bb.setPosition(bb.position() + flatbuffers.SIZE_PREFIX_LENGTH);
+  return (obj || new HandFan()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+length():number {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
+mutate_length(value:number):boolean {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb!.writeInt32(this.bb_pos + offset, value);
+  return true;
+}
+
+static getFullyQualifiedName():string {
+  return 'HandFan';
+}
+
+static startHandFan(builder:flatbuffers.Builder) {
+  builder.startObject(1);
+}
+
+static addLength(builder:flatbuffers.Builder, length:number) {
+  builder.addFieldInt32(0, length, 0);
+}
+
+static endHandFan(builder:flatbuffers.Builder):flatbuffers.Offset {
+  const offset = builder.endObject();
+  return offset;
+}
+
+static createHandFan(builder:flatbuffers.Builder, length:number):flatbuffers.Offset {
+  HandFan.startHandFan(builder);
+  HandFan.addLength(builder, length);
+  return HandFan.endHandFan(builder);
+}
+
+unpack(): HandFanT {
+  return new HandFanT(
+    this.length()
+  );
+}
+
+
+unpackTo(_o: HandFanT): void {
+  _o.length = this.length();
+}
+}
+
+export class HandFanT {
+constructor(
+  public length: number = 0
+){}
+
+
+pack(builder:flatbuffers.Builder): flatbuffers.Offset {
+  return HandFan.createHandFan(builder,
+    this.length
+  );
+}
+}