Fix OrderingConstraint#order forgetting constraints; fix avoidLambdaParams

[smarter]

order takes current as input and returns a constraint set that subsumes both
current and param1 <: param2, but it's an instance method because it relies
on this to determine if current is used linearly such that we can reuse its
backing arrays instead of copying them. However, the implementation of order
mistakenly returned this and called methods on this instead of current.
This lead to issues like #11682 but that was compensated by logic inserted
in ConstraintHandling#addToConstraint which we can now remove.

Fixing this also required fixing an unrelated issue in avoidLambdaParams to
prevent a regression in tests/pos/i9676.scala: we shouldn't avoid a lambda param
under its own binder even if it is in comparedTypeLambdas, the sequence of
operations where this happens is:

[A] =>> List[A] <:< [A] =>> G[A]
  // comparedTypeLambdas ++= ([A] =>> List[A], [A] =>> G[A])
  List[A] <:< G[A]
    [A] =>> List[A] <:< G
      // previously, avoidLambdaParams([A] =>> List[A]) = [A] =>> List[Any],
      // now it leaves the type lambda alone.

We end up checking [A] =>> List[A] <:< G instead of just List <:< G because
of ensureLambdaSub in compareAppliedTypeParamRef. I'm not sure if this is
actually needed, but I decided to not disturb that code too much for now.

[odersky]

Nice catches!