Fix bug which prevented New over type-instantiated aliases.

[odersky]

Previously,

 type Map = HashMap[Int, String]
 new Map

did not work. See test aliasNew.scala for a test.
Formerly this logic handled in Parsers (wrapNew),
but that one does not work for aliastypes.

Review by @DarkDimius

[DarkDimius]

LGTM

[retronym]

% cat sandbox/test.scala
class Map[A, B]
class A {
  type M = Map[Int, String]
}

object Test {
  val a = new A
  val b = new a.M
}

% ../dotty/bin/dotc -Xprint:frontend sandbox/test.scala
result of sandbox/test.scala after frontend:
package <empty> {
  class Map[A, B]()  extends Object() {
    Map$$A
    private[this] type A = Map$$A
    Map$$B
    private[this] type B = Map$$B
  }
  class A()  extends Object() {
    type M = Map[Int, String]
  }
  final lazy module val Test: Test$ = new Test$()
  final module class Test$()  extends Object() { this: Test.type =>
    val a: A = new A()
    val b: Map[Int, String] = new Test.a.M[Int, String]()
  }
}

Shouldn't this be:

val b: Map[Int, String] = new Map[Int, String]()

[retronym]

Probably relatedly, this doesn't typecheck:

class Map[A, B]
class A {
  type M[A] = Map[Int, A]
}

object Test {
  val a = new A
  val b = new a.M // error: wrong number of type parameters for [A, B]()Map[A, B](Test.a.M#<init>); expected: 2
}