Note [Pattern bindings and complete signatures]

Consider
      data T a = MkT a a
      f :: forall a. a->a
      g :: forall b. b->b
      MkT f g = MkT (\x->x) (\y->y)
Here we'll infer a type from the pattern of 'T a', but if we feed in
the signature types for f and g, we'll end up unifying 'a' and 'b'

So we instantiate f and g's signature with TyVarTv skolems
(newMetaTyVarTyVars) that can unify with each other.  If too much
unification takes place, we'll find out when we do the final
impedance-matching check in GHC.Tc.Gen.Bind.mkExport

See Note [TyVarTv] in GHC.Tc.Utils.TcMType

None of this applies to a function binding with a complete
signature, which doesn't use tcInstSig.  See GHC.Tc.Gen.Bind.tcPolyCheck.