[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.php

@@ -0,0 +1,23 @@
+<?php
+// automatically generated by the FlatBuffers compiler, do not modify
+
+class Gadget
+{
+    const NONE = 0;
+    const FallingTub = 1;
+    const HandFan = 2;
+
+    private static $names = array(
+        Gadget::NONE=>"NONE",
+        Gadget::FallingTub=>"FallingTub",
+        Gadget::HandFan=>"HandFan",
+    );
+
+    public static function Name($e)
+    {
+        if (!isset(self::$names[$e])) {
+            throw new \Exception();
+        }
+        return self::$names[$e];
+    }
+}