add override export support

[He-Pin]

Kotlin's delegation can be used when there is an inheritance relation, with current export can't.
With Java we can make use of lombok's @Delegate

scala> class A {
     | def test():Unit = println("testA")
     | }
// defined class A

scala> class B (a:A) extends A {
     | export a.test
     | }
2 |export a.test
  |         ^^^^
  |no eligible member test at B.this.a
  |B.this.a.test cannot be exported because it is already a member of class B

Can do an overriding

scala> class C (a:A) extends A {                                                
     | override def test():Unit = a.test()
     | }

What about add override export support?

scala> class B (a:A) extends A {
     |  override export a.test
     | }

The current implementation can work if the exported member doesn't have concrete definition in the base class.
as:

scala> abstract class A {                                                       
     | def test():Unit}
// defined class A

scala> class B(a:A) extends A {
     | export a.test}
// defined class B

[He-Pin]

refs another request: lampepfl/dotty-feature-requests#85