[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.php

@@ -0,0 +1,92 @@
+<?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 HandFan extends Table
+{
+    /**
+     * @param ByteBuffer $bb
+     * @return HandFan
+     */
+    public static function getRootAsHandFan(ByteBuffer $bb)
+    {
+        $obj = new HandFan();
+        return ($obj->init($bb->getInt($bb->getPosition()) + $bb->getPosition(), $bb));
+    }
+
+    public static function HandFanIdentifier()
+    {
+        return "MOVI";
+    }
+
+    public static function HandFanBufferHasIdentifier(ByteBuffer $buf)
+    {
+        return self::__has_identifier($buf, self::HandFanIdentifier());
+    }
+
+    /**
+     * @param int $_i offset
+     * @param ByteBuffer $_bb
+     * @return HandFan
+     **/
+    public function init($_i, ByteBuffer $_bb)
+    {
+        $this->bb_pos = $_i;
+        $this->bb = $_bb;
+        return $this;
+    }
+
+    /**
+     * @return int
+     */
+    public function getLength()
+    {
+        $o = $this->__offset(4);
+        return $o != 0 ? $this->bb->getInt($o + $this->bb_pos) : 0;
+    }
+
+    /**
+     * @param FlatBufferBuilder $builder
+     * @return void
+     */
+    public static function startHandFan(FlatBufferBuilder $builder)
+    {
+        $builder->StartObject(1);
+    }
+
+    /**
+     * @param FlatBufferBuilder $builder
+     * @return HandFan
+     */
+    public static function createHandFan(FlatBufferBuilder $builder, $length)
+    {
+        $builder->startObject(1);
+        self::addLength($builder, $length);
+        $o = $builder->endObject();
+        return $o;
+    }
+
+    /**
+     * @param FlatBufferBuilder $builder
+     * @param int
+     * @return void
+     */
+    public static function addLength(FlatBufferBuilder $builder, $length)
+    {
+        $builder->addIntX(0, $length, 0);
+    }
+
+    /**
+     * @param FlatBufferBuilder $builder
+     * @return int table offset
+     */
+    public static function endHandFan(FlatBufferBuilder $builder)
+    {
+        $o = $builder->endObject();
+        return $o;
+    }
+}