native_type generated C++ code that couldn't be compiled when used without native_inline.

Related to issue #6770, which never got fixed.

Before, a schema like this would generate code that couldn't be compiled:

native_include "native_type_test_impl.h";

namespace Geometry;

struct Vector3D (native_type:"Native::Vector3D") {
  x:float;
  y:float;
  z:float;
}

table ApplicationData {
  position:Vector3D;
}

root_type ApplicationData;

Using the Vector3D as an array or with the "native_inline" attribute did work as expected.
I've added some code to the C++ generator to fix the issue so it works by itself. It has to create a unique_ptr correctly when unpacking and de-referencing one when packing.

I also added some test cases for the native_type, one with the native_inline attribute and one without to NativeTypeTest.