Inferred Contextual params are less nameable

[som-snytt]

Fixes #24375

The original name can be recovered for messages.

[odersky]

The approach looks workable to me. The test failures seem to come from this part in Message.scala:

    override def ParamRefNameString(param: ParamRef): String =
      seen.record(super.ParamRefNameString(param), param.isInstanceOf[TypeParamRef], param)

Maybe we can try to just turn off seen.record here, i.e. drop the whole def? I am not sure more explanations of parameter names help. But one has to try it out and look at the changes.

[som-snytt]

Just a stop-gap update. I will revisit the parameter disambiguation.

Now it is aware that the written l and the inferred lcontextual$1 are not the same. Or maybe just let it ignore the difference.

def withFile[T](op: (l: caps.Capability) ?-> (f: File^{l}) => T): T =
  op(using caps.cap)(new File)

def test =
  withFile: f =>       // error with level checking, was OK under both schemes before
    val o = Service()
    o.file = f
    o.log

[odersky]

I think @Sporarum's comment on the issue makes sense.

[odersky]

Answering @Sporarum's comment on the issue :
#24375 (comment)

Maybe we should work on the toString function for ContectFunctionParamNames instead of intervening here.