Reflective calls aren't cross compatible with scala 2

[martijnhoekstra]

Reflective calls need language import import scala.reflect.Selectable.reflectiveSelectable in dotty. But that's not cross-compatible with scala 2 without a shim. At https://contributors.scala-lang.org/t/scala-3-migration-guide/4237/7 @smarter suggested making import scala.language.reflectiveCalls make scala 3 behave the same as scala 2.

[sjrd]

This was already implemented in #9420.