swift234

Add [fixit] to suggest Self.member instead of Proto.member

Protocol types cannot provide an implementation of their members — when
one is looked up on a protocol in that protocol’s context, we can
suggest using the dynamic `Self` type.

```
extension StaticP {
  func bar() {
    _ = StaticP.foo(a:)  // suggest `Self.foo(a:)`
  }
}
```