There is a general invariant that the constraint solver doesn't see
TyVars, only TcTyVars.  But when checking the generic-default
signature of a class, we called checkValidType on the generic-default
type, which had the class TyVar free. That in turn meant that it wasn't
considered during flattening, which led to the error reported in
Trac #11608.

The fix is simple: call checkValidType on the /closed/ type. Easy.

While I was at it, I added a bunch of ASSERTs about the TcTyVar
invariant.