[D] Fix occasional undefined behaviour with inheritance hierarchies

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.

Examples/test-suite/cpp11_std_unique_ptr.i

@@ -10,6 +10,7 @@
 %inline %{
 #include <memory>
 #include <string>
+//#include <iostream>
 #include "swig_examples_lock.h"
 
 class Klass {
@@ -53,8 +54,18 @@ struct KlassInheritance : virtual Klass {
   }
 };
 
+std::string useKlassRawPtr(Klass* k) {
+//  std::cout << "useKlassRawPtr " << std::hex << (Klass*)k << std::endl;
+  std::string s(k->getLabel());
+//  std::cout << "useKlassRawPtr string: " << s << std::endl;
+  return s;
+}
+
 std::string takeKlassUniquePtr(std::unique_ptr<Klass> k) {
-  return std::string(k->getLabel());
+//  std::cout << "takeKlassUniquePtr " << std::hex << (Klass*)k.get() << std::endl;
+  std::string s(k->getLabel());
+//  std::cout << "takeKlassUniquePtr string: " << s << std::endl;
+  return s;
 }
 
 bool is_nullptr(Klass *p) {