Start emitting stubs for unimplemented designated initializers of the superclass.

When a subclass does not implement a designated initializer of its
superclass, introduce a stub initializer that simply traps. Such stubs
cannot be invoked directly using Swift syntax, but can be invoked
through the Objective-C runtime and from Objective-C code. Catch such
errors rather than allowing them to violate the memory safety of the
language.

Note that we're currently using cond_fail to trap; this will be
improved in the future,


Swift SVN r14839

lib/Serialization/Serialization.cpp

@@ -873,6 +873,12 @@ static bool shouldSerializeMember(Decl *D) {
   case DeclKind::EnumCase:
     return false;
 
+  case DeclKind::Constructor: {
+    // Never serialize a constructor with a stub implementation.
+    auto ctor = cast<ConstructorDecl>(D);
+    return !ctor->hasStubImplementation();
+  }
+
   case DeclKind::EnumElement:
   case DeclKind::Protocol:
   case DeclKind::Destructor:
@@ -886,7 +892,6 @@ static bool shouldSerializeMember(Decl *D) {
   case DeclKind::Class:
   case DeclKind::Var:
   case DeclKind::Func:
-  case DeclKind::Constructor:
     return true;
   }
 }