Particularly when using virtual inheritance as the pointers weren't
correctly upcast from derived class to base class when stored in the
base's proxy class.
Fixes commented out test code in cpp11_std_unique_ptr_runme
and li_std_auto_ptr_runme D tests.
This works by transferring ownership of the underlying C++ memory
from the target language proxy class to an instance of the unique_ptr
which is passed to the wrapped function via std::move.
The proxy class has a new swigRelease() method which sets the
underlying C++ pointer for the proxy class to null, so working
in much the same way as std::unique_ptr::release(). Any attempt at
using the proxy class will be the same as if the delete() function
has been called on the proxy class. That is, using a C++ null pointer,
when a non-null pointer is usually expected.
This commit relies on the previous commit that uses std::move
on the temporary variable used for the wrapped function's input parameter
as std::unique_ptr is not copyable, it has move-only semantics.
