Class-based REPL does not support defining value classes

Classes extending AnyVal cannot be defined in a REPL started with the `-Yrepl-class-based` option.

E.g.
```scala
scala> case class Foo(x: Int) extends AnyVal
error: value class may not be a member of another class
       case class Foo(x: Int) extends AnyVal
                  ^
```

without the -Yrepl-class-based option, everything works as expected.