Add CodingKey and *codable derived conformance

* Allow CodingKey conformance to be automatically derived for enums
  which have no raw type (with no associated values) and which have
  a raw type of String or Int.
* Allow Encodable and Decodable conformance to be automatically derived
  for classes and structs with Encodable/Decodable properties
* Add initial unit tests for verifying derived conformance

test/decl/protocol/special/coding/class_codable_inheritance.swift

@@ -0,0 +1,59 @@
+// RUN: %target-typecheck-verify-swift -verify-ignore-unknown
+
+// REQUIRES: objc_interop
+
+import Foundation
+
+class SimpleClass : Codable {
+  var x: Int = 1
+  var y: Double = .pi
+  static var z: String = "foo"
+}
+
+// The synthesized CodingKeys type should not be accessible from outside the
+// class.
+let _ = SimpleClass.CodingKeys.self // expected-error {{'CodingKeys' is inaccessible due to 'private' protection level}}
+
+// Classes which inherit from classes that are codable should synthesize Codable
+// conformance as well.
+class SimpleChildClass : SimpleClass {
+  var w: Bool = true
+
+  // NOTE: These tests will fail in the future as Codable classes are updated
+  //       to derive Codable conformance instead of inheriting their
+  //       superclass's. Classes currently inherit their parent's Codable
+  //       conformance and we never get the chance to derive a CodingKeys
+  //       type, nor overridden methods.
+
+  // These lines have to be within the SimpleChildClass type because
+  // CodingKeys should be private.
+  func foo() {
+    // They should receive a synthesized CodingKeys enum.
+    // NOTE: This expected error will need to be removed in the future.
+    let _ = SimpleChildClass.CodingKeys.self // expected-error {{'CodingKeys' is inaccessible due to 'private' protection level}}
+
+    // The enum should have a case for each of the vars.
+    // NOTE: This expectedxerror will need to be removed in the future.
+    let _ = SimpleChildClass.CodingKeys.w // expected-error {{'CodingKeys' is inaccessible due to 'private' protection level}}
+
+    // Inherited vars should not be part of the CodingKeys enum.
+    // NOTE: This expected error will need to be updated in the future.
+    //       Should be `type 'SimpleClass.CodingKeys' has no member 'x'`
+    let _ = SimpleChildClass.CodingKeys.x // expected-error {{'CodingKeys' is inaccessible due to 'private' protection level}}
+
+    // NOTE: This expected error will need to be updated in the future.
+    //       Should be `type 'SimpleClass.CodingKeys' has no member 'y'`
+    let _ = SimpleChildClass.CodingKeys.y // expected-error {{'CodingKeys' is inaccessible due to 'private' protection level}}
+  }
+}
+
+// These are wrapped in a dummy function to avoid binding a global variable.
+func foo() {
+  // They should receive synthesized init(from:) and an encode(to:).
+  let _ = SimpleChildClass.init(from:)
+  let _ = SimpleChildClass.encode(to:)
+
+  // The synthesized CodingKeys type should not be accessible from outside the
+  // class.
+  let _ = SimpleChildClass.CodingKeys.self // expected-error {{'CodingKeys' is inaccessible due to 'private' protection level}}
+}