Issue Description:

`CaseIterable` was introduced in Swift 4.2, yet when compiling projects with the Swift 4.2 compiler in Swift 4.0 compatibility mode, the protocol is available. This is creating problems with a shim `CaseIterable` protocol I would like to declare only in < Swift 4.2.

#if swift(>=4.2)
#else
public protocol CaseIterable {
    static var allCases: [Self] { get }
}
#endif

When running Swift 4.1 compiler, `CaseIterable` is usable. When running Swift 4.2 compiler in 4.0 compat mode, `CaseIterable` becomes ambiguous.

Using the placeholder #compiler directive, I can get this to work.

#if swift(>=4.1.50)
#else
public protocol CaseIterable {
    static var allCases: [Self] { get }
}
#endif

This solution works fine for me, but I wanted to notify you all in case this is unexpected behavior.

Thanks and keep up the great work on Swift 4.2!