Clang refuses to default initialize an instance of a class that does not
have a default constructor. Fix the check by adding default
constructors. Don't use brace initialization like it is proposed in the
error message. We want to test the override support independent from
the support for brace initialization.
This is important for two reasons:
1. A compiler might warn about a class that has a virtual member
function but no virtual destructor. We don't want to treat the feature
as incomplete in this case.
2. MSVC10 supports the override identifier except on destructors. In
this case, the feature really is incomplete and we want to detect it as
such.