Issue Description:

This little code snippet will trigger a warning underlining `B`. Obviously, there shouldn't be a problem here as long as all occurrences of `Foo` in `A` are at least internal (or stricter). Otherwise, they are simply downgraded to the same access level as the type that is involved.

Thread

internal class Foo {} 

public class A<T> {} 

public class B: A<Foo> {} // Class should not be declared public because its superclass is internal

I understand this is simply a warning, but I think it is understandable they can be annoying when appearing in hundreds from pods and external frameworks without a significant reason. I would like to request not to show this warning unless it bears important information (i.e. that can lead to unsafe code.) or if the code doesn't involve any access level downgrading because of the inheritance, like in my case.

In contrast, a case that deserves a warning or even an error (warning if t and init aren't explicitly public, otherwise error)

internal class Foo {} 

public class A<T> {
    public var t: T 
    public init(_ t: T) { self.t = t }
}
public class B: A<Foo> {}