Detect missing parent in clone at compile time

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=2511

Reported by Martin Stjernholm mast@roxen.com

Consider the following program:

    class X {
      int i;
      class Y {int f() {return i;}}
    }

    int main() {
      X.Y y = X.Y();
    }

It will compile, but will fail at run time with "Parent lost, cannot clone program". Besides the somewhat misleading error message, I think this case ought to be catched already at compile time.