Fix scala 3.8 regression

[jchyb]

Starting with scala.3.8, standard library will now be compiled with scala 3, instead of reusing scala 2 artifacts. This means that the way certain symbols are represented will change. For this project, we've run into an issue, where for:

sealed abstract class Infinite
val Undefined: Infinite = new Infinite {}

scala 2 would generate a <local child> symbol to assign to the child anonymous class, and scala 3 would intentionally reuse the parent symbol with the same goal - both behaviors are intended for pattern matcher to be able to give exhaustivity errors.
In this fix here, we handle the scala 3 local child the same way we did for the scala 2 counterpart.
Related discussion: scala/scala3#24598

[plokhotnyuk]

@jchyb Thanks a lot for explanations and the contribution!