[Runtime] Add missing FakeUnavailableSwiftDylib.swift file.

Author: mikeash

test/Interpreter/Inputs/FakeUnavailableSwiftDylib.swift

@@ -0,0 +1,9 @@
+@available(OSX 1066.0, iOS 1066.0, watchOS 1066.0, tvOS 1066.0, *)
+public protocol UnavailableSwiftProtocol {
+  func someMethod()
+}
+
+@available(OSX 1066.0, iOS 1066.0, watchOS 1066.0, tvOS 1066.0, *)
+public class UnavailableSwiftClass {
+  func someMethod() {}
+}