Report error in Setup for illegal capture refs

[bracevac]

Fixes #21868

[bracevac]

@noti0na1 I think it'd be good to merge this into #21875

[noti0na1]

Sure! I will take a look.

[bracevac]

The test case for this is tied to the paths to "capture set type members" that your PR fixes:

import language.experimental.captureChecking
import caps._

trait Abstract:
    type C <: CapSet
    def boom(): Unit^{this.C^}

class Concrete extends Abstract:
    type C = Nothing
    def boom() = () 

now yields

-- Error: local/i21868.scala:10:14 ---------------------------------------------
10 |    def boom() = () 
   |              ^
   |              Illegal capture reference: Nothing
Illegal capture reference: Nothing
-- Error: local/i21868.scala:6:22 ----------------------------------------------
6 |    def boom(): Unit^{this.C^}
  |                      ^^^^^^^
  |          Abstract.this.C cannot be tracked since its capture set is empty

And I think the second one shouldn't be reported.

Edit: removed an unnecessary class definition

[noti0na1]

I think the second error is valid, because the capture set of the upper bound of type C <: CapSet is empty.

[noti0na1]

It seems the function is called twice on the same type at different places, so the error is reported twice.

[bracevac]

Merged with #21875