Issue Description:

It’s common for folks to reach for existentials in places we don’t allow them, e.g.,

struct ContainerView: View {
    var childView: View   // error: Protocol ‘View’ can only be used as a generic constraint because it has Self or associated type requirements
  }

In some common scenarios, such as stored properties of a struct, cases of an enum, or parameters of a function, the answer is to parameterize the enclosing entity, e.g.,

  struct ContainerView<V: View>: View {
    var childView: V
  }

Can we suggest such a transformation as a Fix-It to help the user along?