Add regression test for https://bugs.swift.org/browse/SR-14668

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr14668.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+class A { }
+class B<T> : A { }
+class C {
+  func bar(x : (A?) -> Void) { }
+}
+class D<T> : C {
+  override func bar(x : (B<T>?) -> Void) { }
+}