Restore AllCases Default

While we cannot yet remove the synthesis of AllCases without some extra
coordination with the SDKs, we can at least leave the default behind
since it is always going to be correct, if a little duplicative.

Author: CodaFi

stdlib/public/core/CompilerProtocols.swift

@@ -249,7 +249,7 @@ extension RawRepresentable where RawValue: Hashable, Self: Hashable {
 /// demonstrates this automatic implementation.
 public protocol CaseIterable {
   /// A type that can represent a collection of all values of this type.
-  associatedtype AllCases: Collection
+  associatedtype AllCases: Collection = [Self]
     where AllCases.Element == Self
 
   /// A collection of all values of this type.

test/IDE/complete_override.swift

@@ -896,11 +896,10 @@ struct SynthesizedConformance3: Hashable {
 enum SynthesizedConformance4: CaseIterable {
   case a, b, c, d
   #^OVERRIDE_SYNTHESIZED_4^#
-// OVERRIDE_SYNTHESIZED_4: Begin completions, 4 items
+// OVERRIDE_SYNTHESIZED_4: Begin completions, 3 items
 // OVERRIDE_SYNTHESIZED_4-DAG: Decl[InstanceVar]/Super/IsSystem:       var hashValue: Int
 // OVERRIDE_SYNTHESIZED_4-DAG: Decl[InstanceMethod]/Super/IsSystem:    func hash(into hasher: inout Hasher) {|};
 // OVERRIDE_SYNTHESIZED_4-DAG: Decl[StaticVar]/Super/IsSystem:         static var allCases: [SynthesizedConformance4];
-// OVERRIDE_SYNTHESIZED_4-DAG: Decl[AssociatedType]/Super/IsSystem:    typealias AllCases = {#(Type)#};
 }
 
 class SynthesizedConformance5: SynthesizedConformance2 {