Fix i19315: avoid calling addOuterRefs when the actual type is box-adapted

[Linyxus]

Fixes #19315.

The unsound code:

import language.experimental.captureChecking
trait Logger
case class Boxed[T](unbox: T)

// a horrible function, but it typechecks
def magic(l: Logger^): Logger =
  class Foo:
    def foo: Boxed[Logger^{this}] =
      // for the following line to typecheck
      //   the capture checker assumes {l} <: {this}
      Boxed[Logger^{this}](l)
  val x = new Foo
  val y = x.foo.unbox  // y: Logger^{x}
  val z: Logger = y  // now the capability becomes pure
  z

The magic function typechecks before this fix. It casts a capability to a pure value, which is clearly unsound. The crux of the problem is Boxed[Logger^{this}](l), which relies on the subcapturing relation {l} <: {this} enabled by the trick implemented addOuterRefs.

addOuterRefs augment a capture set that contains a self reference (like {this}) with all outer references reachable from this. In this case, the augmented set is {this, l}. The reasoning is that, any captured outer references in the class can be rewritten into a field, thus being a subcapture of {this}:

class Foo:
  val this_l: Logger^{l} = l
  def foo: Boxed[Logger^{this}] = Boxed(this.this_l)

But the problem with this unsound example is that, the reference to l is boxed, so l is not captured by the class. Therefore, the above rewriting mismatches with the actual situation.

This PR proposes a simple fix, which simply disable addOuterRefs when box adaptation happens. This is of course imprecise, but all tests pass with this fix.

[Linyxus]

I'll look into the failed test first.

[Linyxus]

The previously-failing test seems to be a random incident. It passes after re-running. This PR is ready for review.