[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/union_vector.ts

@@ -1,5 +1,8 @@
 export { Attacker, AttackerT } from './attacker';
 export { BookReader, BookReaderT } from './book-reader';
 export { Character, unionToCharacter, unionListToCharacter } from './character';
+export { FallingTub, FallingTubT } from './falling-tub';
+export { Gadget, unionToGadget, unionListToGadget } from './gadget';
+export { HandFan, HandFanT } from './hand-fan';
 export { Movie, MovieT } from './movie';
 export { Rapunzel, RapunzelT } from './rapunzel';