Unify the end-of-class code paths used by the parser and template

instantiation, to ensure that we mark class template specilizations as
abstract when we need to and perform checking of abstract classes.

Also, move the checking that determines whether we are creating a
variable of abstract class type *after* we check whether the type is
complete. Otherwise, we won't see when we have an abstract class
template specialization that is implicitly instantiated by this
declaration. This is the "something else" that Sebastian had noted
earlier.

llvm-svn: 90467