Do not expand a derive invocation when derive is not allowed

1. Change the return type of `expand_invoc()` and its subroutines to
   `Option<Expansion>` from `Expansion`.
2. Return `None` when expanding a derive invocation if the item cannot
   have derive on it (in `expand_derive_invoc()`).

src/test/compile-fail/derive-on-trait-item-or-impl-item.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+trait Foo {
+    #[derive(Clone)]
+    //~^ ERROR `derive` may only be applied to structs, enums and unions
+    type Bar;
+}
+
+impl Bar {
+    #[derive(Clone)]
+    //~^ ERROR `derive` may only be applied to structs, enums and unions
+    fn bar(&self) {}
+}
+
+fn main() {}