For type synonyms, we need to check that if the RHS has
kind Constraint, then we have -XConstraintKinds.  For
some reason this was done in checkValidType, but it makes
more sense to do it in checkValidTyCon.

I can't remember quite why I made this change; maybe it fixes
a Trac ticket, but if so I forget which.  But it's a modest
improvement anyway.