Referring to types from other namespaces in C++ now works correctly.

Previously, it would ignore the fact that the type comes from a
different namespace. Now they are pre-declared in their own namespace,
and referenced with a qualified name if necessary.

Bug: 16851682
Change-Id: I5cb625b86d28e7436b9e93c70a0fa16a600d9884
Tested: on Linux

tests/monster_test_generated.h

@@ -5,9 +5,19 @@
 
 #include "flatbuffers/flatbuffers.h"
 
+namespace MyGame {
+namespace OtherNameSpace {
+struct Unused;
+}  // namespace OtherNameSpace
+}  // namespace MyGame
+
 namespace MyGame {
 namespace Example {
 
+struct Test;
+struct Vec3;
+struct Monster;
+
 enum {
   Color_Red = 1,
   Color_Green = 2,
@@ -35,10 +45,6 @@ inline const char *EnumNameAny(int e) { return EnumNamesAny()[e]; }
 
 bool VerifyAny(const flatbuffers::Verifier &verifier, const void *union_obj, uint8_t type);
 
-struct Test;
-struct Vec3;
-struct Monster;
-
 MANUALLY_ALIGNED_STRUCT(2) Test {
  private:
   int16_t a_;