Only allow access to inline args at level 1

[nicolasstucki]

If it is needed at level 1 it can be explicitly lifted.
This simplifies the rules that users need to know and
the implementation of the phase.

[Blaisorblade]

That's the change we discussed time ago right? 👍 from me.

[nicolasstucki]

Not exactly what we discussed, but it is related. I still want to do the one we discussed but it is a discussion for another PR.

[odersky]

I did not understand why the restriction was made?

Anyway, if it was made, the PMP document has to be updated accordingly. Not just change the wording, but motivate it and explain it there so that a reader can understand the restriction.

But I would prefer if we did not have to do it.

[nicolasstucki]

Thinking about it, the simplification I wanted to make only works for inline parameters, not for inline vals. Though this experiment showed me some vulnerability in the implementation of liftValue. I fix for that in #4777.