Fix #23223: Add handling for classes deriving from Capability during Setup

[noti0na1]

Fix #23223

Updated the Setup logic to handle classes deriving from Capability by adding a fresh capability.

[noti0na1]

Error:

import language.experimental.captureChecking

class Cap extends caps.Capability:
  def use[T](body: Cap ?=> T) = body(using this)

-- [E007] Type Mismatch Error: tests/pos-custom-args/captures/i20237.scala:4:43 
4 |  def use[T](body: Cap ?=> T) = body(using this)
  |                                           ^^^^
  |Found:    (Cap.this : Cap^{cap.rd})
  |Required: Cap^{cap².rd}
  |
  |where:    cap  is the universal root capability
  |          cap² is a fresh root capability created in method use when checking argument to parameter x$0 of method apply

[noti0na1]

cc @Linyxus

[odersky]

I have tweaked the previous scheme so that we only add one capture set variable to a self type. Doing a ++ is correct, but creates two variables, one for the original set and one for the result of the ++.

[noti0na1]

but creates two variables, one for the original set and one for the result of the ++

Oh, I didn't realize there is another variable