Issue Description:

In Swift 3, we cleaned up the argument label model for most declarations, but enum cases got left behind. If you have cases with labeled payloads:

enum Foo {
  case bar(bas: Int)
  case zim(zang: Int, zung: String)
}

then the cases are still formally declarations named bar and zim, with labeled tuples as payload types. They should behave more consistently with other declarations and have the labels become part of the declaration name.