TagMacro: Dotty: return correct closestClass in Tags

[neko-kai]

Reported in #116 (comment)

Dotty version currently always uses classOf[Any] as the closestClass instead of finding the LUB of a type as the Scala 2 version does.

[neko-kai]

Note that it's not enought to splice classOf[T], we should mirror behavior of closestClass for Scala 2:

  @inline
  private[this] final def closestClass(properTypeStrongCtor: Type): c.Expr[Class[_]] = {
    // unfortunately .erasure returns trash for intersection types
    val tpeLub = properTypeStrongCtor match {
      case r: RefinedTypeApi => lub(r.parents)
      case _ => properTypeStrongCtor
    }
    c.Expr[Class[_]](q"_root_.scala.Predef.classOf[${tpeLub.erasure}]")
  }

That is, the complexity is essentially in finding analogue of c.universe.lub function in the new tasty-reflect API and extracting the LUB for intersection type and using the default behavior of classOf for all other cases.