AST: Skip stub initializers when printing module interfaces

When a derived class does not inherit a designated initializer from
its base class, we override the designated initializer's vtable
entry with a stub which traps with a fatal error.

The stub cannot be called and clients do not need to be aware of
its existence, so don't print it at all in the module interface.

Fixes rdar://problem/71122015 / https://bugs.swift.org/browse/SR-13832.

test/ModuleInterface/stub-init.swift

@@ -0,0 +1,22 @@
+// RUN: %target-swift-frontend -typecheck -swift-version 5 -emit-module-interface-path - -module-name test %s | %FileCheck %s
+
+public class Base {
+  public init(x: Int) {}
+}
+
+public class Derived: Base {
+  public init(z: Int) {
+    super.init(x: z)
+  }
+}
+
+// CHECK-LABEL: public class Base {
+// CHECK-NEXT:    public init(x: Swift.Int)
+// CHECK-NEXT:    {{(@objc )?}}deinit
+// CHECK-NEXT: }
+
+// CHECK-LABEL: public class Derived : test.Base {
+// CHECK-NEXT:    public init(z: Swift.Int)
+// CHECK-NEXT:    {{(@objc )?}}deinit
+// CHECK-NEXT: }
+