Type synonyms that don't mention one of their type parameters on the 
RHS are a pain in the neck.  This patch fixes a long-standing bug (that
simply has not appeared before) in that exprType could return a type
mentioning an existentially-quantified variable (in one of those ignored
argument positions).

See CoreUtils Note [Existential variables and silly type synonyms]

The fix is not entirely beautiful, but it works, and is very localised.