[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/FallingTub.php

@@ -0,0 +1,41 @@
+<?php
+// automatically generated by the FlatBuffers compiler, do not modify
+
+use \Google\FlatBuffers\Struct;
+use \Google\FlatBuffers\Table;
+use \Google\FlatBuffers\ByteBuffer;
+use \Google\FlatBuffers\FlatBufferBuilder;
+
+class FallingTub extends Struct
+{
+    /**
+     * @param int $_i offset
+     * @param ByteBuffer $_bb
+     * @return FallingTub
+     **/
+    public function init($_i, ByteBuffer $_bb)
+    {
+        $this->bb_pos = $_i;
+        $this->bb = $_bb;
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function GetWeight()
+    {
+        return $this->bb->getInt($this->bb_pos + 0);
+    }
+
+
+    /**
+     * @return int offset
+     */
+    public static function createFallingTub(FlatBufferBuilder $builder, $weight)
+    {
+        $builder->prep(4, 4);
+        $builder->putInt($weight);
+        return $builder->offset();
+    }
+}