Implement DR257 / fix PR16659:
  A constructor for an abstract class does not call constructors for virtual
  base classes, so it is not an error if no initializer is present for the
  virtual base and the virtual base cannot be default initialized.

Also provide a (disabled by default, for now) warning for the case where a
virtual base class's initializer is ignored in an abstract class's constructor,
and address a defect in DR257 where it was not carried through to C++11's rules
for implicit deletion of special member functions.

Based on a patch by Maurice Bos.