Issue Description:

class SomeClass {
}

fileprivate extension SomeClass {
    fileprivate var someProperty: Int {
        return 1
    }
}

In this code, the fileprivate on the property is redundant with the one on the extension. Since access modifiers on extensions are an opt-in feature, anyone using it probably didn't intend to write the access in both places. We can warn about this.

Note that this does not apply to private, since private extension is currently equivalent to fileprivate extension, but private on the members means "within this file, visible inside this type and all of its extensions".