Add missing classof().

Somewhat troublingly, without this implemented, the check inside
isa_impl<> would silently use the parent's `classof()` when determining
whether it was okay to downcast from the parent to the child!

Bug analysis:

A build failure after removing the parent's `classof()` initially
alerted me to the bug, after which a little bit of thinking and reading
of the code identified the root cause.

The compiler could be made to prevent this bug from happening if there
were a way to ensure that in the code

    template <typename To, typename From, typename Enabler = void>
    struct isa_impl {
      static inline bool doit(const From &Val) {
        return To::classof(&Val);
      }
    };

that `To::classof` is actually inside the class `To`, and not in a base
class. I am not aware of a way to check this in C++. If there is a means
to perform that check, please bring it up on the list and this will be
fixed.

There is a high likelihood that there are other instances of this same
bug in the codebase.

llvm-svn: 165769