Issue 5450 - Don't generate default ctor if base class lacks default ctor

http://d.puremagic.com/issues/show_bug.cgi?id=5450

This effectively reverses changes introduced in https://github.com/D-Programming-Language/dmd/pull/1336 because they were unnecessary. A default ctor shouldn't be generated unless the base class has a default ctor. This will also improve the error message.