export should ignore overriden methods from Object

[liufengyun]

minimized code

class A {
  override def toString: String = "A"
}

class B(a: A) {
  export a._
}

6 |  export a._
  |           ^
  |         error overriding method toString in class Any of type (): String;
  |           method toString of type => String needs override modifier
one error found

expectation

The line export a._ should not export the method A.toString.

The specification says:

A member is eligible if all of the following holds:

- its owner is not a base class of the class(*) containing the export clause,
- it is accessible at the export clause,
- it is not a constructor, nor the (synthetic) class part of an object,
- it is a given instance (or an old-style implicit value) if and only if the export is tagged with given.

Maybe the first clause can be strengthened:

(1) its owner is not a base class of the class(*) containing the export clause
    - if the member overrides a method "m" in a base class, 
      then perform the same check (1) for "m".