Move the constraint-kind validity check
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.
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