Disallow open modifier on traits and abstract classes

[smarter]

As https://dotty.epfl.ch/docs/reference/other-new-features/open-classes.html mentions:

Traits or abstract classes are always open, so open is redundant for them.

But right now open trait Foo and open abstract class Foo compile without error, they should at least emit a warning, like final object Foo already does.

[smarter]

We should probably also disallow open on local classes since there's no way to extend a local class from a different file anyway:

object Test {
  def foo: Any = {
    open class Bar
    new Bar
  }
}

[megri]

I'm working on the first part of this (excluding local classes for now).
The check is being done in Desugar.scala with report.warning. Am I on the right track?

[smarter]

I think that's fine, it might also be possible to do it in the Parser but that's not necessarily better.

[odersky]

The check should be integrated in Checking.checkWellFormed. That's the place we check all the other flag combinations.