[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/HandFan.kt

@@ -0,0 +1,51 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import java.nio.*
+import kotlin.math.sign
+import com.google.flatbuffers.*
+
+@Suppress("unused")
+@ExperimentalUnsignedTypes
+class HandFan : Table() {
+
+    fun __init(_i: Int, _bb: ByteBuffer)  {
+        __reset(_i, _bb)
+    }
+    fun __assign(_i: Int, _bb: ByteBuffer) : HandFan {
+        __init(_i, _bb)
+        return this
+    }
+    val length : Int
+        get() {
+            val o = __offset(4)
+            return if(o != 0) bb.getInt(o + bb_pos) else 0
+        }
+    fun mutateLength(length: Int) : Boolean {
+        val o = __offset(4)
+        return if (o != 0) {
+            bb.putInt(o + bb_pos, length)
+            true
+        } else {
+            false
+        }
+    }
+    companion object {
+        fun validateVersion() = Constants.FLATBUFFERS_2_0_0()
+        fun getRootAsHandFan(_bb: ByteBuffer): HandFan = getRootAsHandFan(_bb, HandFan())
+        fun getRootAsHandFan(_bb: ByteBuffer, obj: HandFan): HandFan {
+            _bb.order(ByteOrder.LITTLE_ENDIAN)
+            return (obj.__assign(_bb.getInt(_bb.position()) + _bb.position(), _bb))
+        }
+        fun createHandFan(builder: FlatBufferBuilder, length: Int) : Int {
+            builder.startTable(1)
+            addLength(builder, length)
+            return endHandFan(builder)
+        }
+        fun startHandFan(builder: FlatBufferBuilder) = builder.startTable(1)
+        fun addLength(builder: FlatBufferBuilder, length: Int) = builder.addInt(0, length, 0)
+        fun endHandFan(builder: FlatBufferBuilder) : Int {
+            val o = builder.endTable()
+            return o
+        }
+    }
+}