Fixed namespace code generation for C++

The code generator was assuming all declarations for the current
file sit in the same namepace. Now uses the "on demand" namespace
switching we had for the forward declarations.

Also fixed a bug related to namespace lookup.

Change-Id: Ib54a3efbc752cbb9590302fa0707c0c73448db3d
Tested: on Linux.

tests/namespace_test/NamespaceA/SecondTableInA.php

@@ -0,0 +1,82 @@
+<?php
+// automatically generated, do not modify
+
+namespace NamespaceA;
+
+use \Google\FlatBuffers\Struct;
+use \Google\FlatBuffers\Table;
+use \Google\FlatBuffers\ByteBuffer;
+use \Google\FlatBuffers\FlatBufferBuilder;
+
+class SecondTableInA extends Table
+{
+    /**
+     * @param ByteBuffer $bb
+     * @return SecondTableInA
+     */
+    public static function getRootAsSecondTableInA(ByteBuffer $bb)
+    {
+        $obj = new SecondTableInA();
+        return ($obj->init($bb->getInt($bb->getPosition()) + $bb->getPosition(), $bb));
+    }
+
+    /**
+     * @param int $_i offset
+     * @param ByteBuffer $_bb
+     * @return SecondTableInA
+     **/
+    public function init($_i, ByteBuffer $_bb)
+    {
+        $this->bb_pos = $_i;
+        $this->bb = $_bb;
+        return $this;
+    }
+
+    public function getReferToC()
+    {
+        $obj = new TableInC();
+        $o = $this->__offset(4);
+        return $o != 0 ? $obj->init($this->__indirect($o + $this->bb_pos), $this->bb) : 0;
+    }
+
+    /**
+     * @param FlatBufferBuilder $builder
+     * @return void
+     */
+    public static function startSecondTableInA(FlatBufferBuilder $builder)
+    {
+        $builder->StartObject(1);
+    }
+
+    /**
+     * @param FlatBufferBuilder $builder
+     * @return SecondTableInA
+     */
+    public static function createSecondTableInA(FlatBufferBuilder $builder, $refer_to_c)
+    {
+        $builder->startObject(1);
+        self::addReferToC($builder, $refer_to_c);
+        $o = $builder->endObject();
+        return $o;
+    }
+
+    /**
+     * @param FlatBufferBuilder $builder
+     * @param int
+     * @return void
+     */
+    public static function addReferToC(FlatBufferBuilder $builder, $referToC)
+    {
+        $builder->addOffsetX(0, $referToC, 0);
+    }
+
+    /**
+     * @param FlatBufferBuilder $builder
+     * @return int table offset
+     */
+    public static function endSecondTableInA(FlatBufferBuilder $builder)
+    {
+        $o = $builder->endObject();
+        return $o;
+    }
+}