Use ClassDecl::ForeignKind to model Clang's objc_runtime_visible.

We're now correctly checking for inheritance, adding @objc methods,
and adding @objc protocols for both CF types and objc_runtime_visible
classes (those without visible symbols). The latter is used for some
of the types in Dispatch, which has exposed some of the classes that
were considered implementation details on past OSs.

We still don't properly implement using 'as?' to check conformance to
a Swift protocol for a CF or objc_runtime_visible type, but we can do
that later.

rdar://problem/26850367

test/Interpreter/SDK/cf_extensions.swift

@@ -1,9 +1,10 @@
-// RUN: %target-run-simple-swift | FileCheck %s
+// RUN: %target-run-simple-swift
 // REQUIRES: executable_test
 
 // REQUIRES: objc_interop
 
 import Foundation
+import StdlibUnittest
 
 #if os(OSX)
 import AppKit
@@ -30,13 +31,47 @@ extension CGColor {
   var b: CGFloat { return components![2] }
 }
 
-let pink = CGColor.create(colorSpace: .deviceRGB(),
-                          components: [1.0, 0.5, 0.25, 1.0])
+var CFTestSuite = TestSuite("CFExtensions")
 
-// CHECK: 1.0
-print(pink.r)
-// CHECK-NEXT: 0.5
-print(pink.g)
-// CHECK-NEXT: 0.25
-print(pink.b)
+CFTestSuite.test("methods") {
+  let pink = CGColor.create(colorSpace: .deviceRGB(),
+                            components: [1.0, 0.5, 0.25, 1.0])
+  expectEqual(1.0, pink.r)
+  expectEqual(0.5, pink.g)
+  expectEqual(0.25, pink.b)
+}
 
+protocol SwiftProto {
+  func doTheThing() -> AnyObject
+}
+extension CGColor: SwiftProto {
+  func doTheThing() -> AnyObject { return self }
+}
+
+func callTheThing<T: SwiftProto>(_ instance: T) -> AnyObject {
+  return instance.doTheThing()
+}
+
+CFTestSuite.test("protocols") {
+  let pink = CGColor.create(colorSpace: .deviceRGB(),
+                            components: [1.0, 0.5, 0.25, 1.0])
+  expectTrue(pink === pink.doTheThing())
+
+  let protoObj: SwiftProto = pink
+  expectTrue(pink === protoObj.doTheThing())
+
+  expectTrue(pink === callTheThing(pink))
+}
+
+CFTestSuite.test("protocols/downcast")
+    .xfail(.always("unimplemented"))
+    .code {
+  let pink = CGColor.create(colorSpace: .deviceRGB(),
+                            components: [1.0, 0.5, 0.25, 1.0])
+  let opaquePink: AnyObject = pink
+  let downcasted = opaquePink as? SwiftProto
+  expectNotEmpty(downcasted)
+  expectTrue(pink === downcasted!.doTheThing())
+}
+
+runAllTests()