[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/gadget.ts

@@ -0,0 +1,37 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import { FallingTub, FallingTubT } from './falling-tub';
+import { HandFan, HandFanT } from './hand-fan';
+
+
+export enum Gadget{
+  NONE = 0,
+  FallingTub = 1,
+  HandFan = 2
+}
+
+export function unionToGadget(
+  type: Gadget,
+  accessor: (obj:FallingTub|HandFan) => FallingTub|HandFan|null
+): FallingTub|HandFan|null {
+  switch(Gadget[type]) {
+    case 'NONE': return null; 
+    case 'FallingTub': return accessor(new FallingTub())! as FallingTub;
+    case 'HandFan': return accessor(new HandFan())! as HandFan;
+    default: return null;
+  }
+}
+
+export function unionListToGadget(
+  type: Gadget, 
+  accessor: (index: number, obj:FallingTub|HandFan) => FallingTub|HandFan|null, 
+  index: number
+): FallingTub|HandFan|null {
+  switch(Gadget[type]) {
+    case 'NONE': return null; 
+    case 'FallingTub': return accessor(index, new FallingTub())! as FallingTub;
+    case 'HandFan': return accessor(index, new HandFan())! as HandFan;
+    default: return null;
+  }
+}
+