rustc: Add lint for obsolete attributes

This also moves `#[auto_{en,de}code]` checker from syntax to lint.

src/test/compile-fail/deprecated-auto-code.rs

@@ -1,15 +0,0 @@
-// Copyright 2013 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.
-
-#[auto_encode] //~ ERROR: `#[auto_encode]` is deprecated
-#[auto_decode] //~ ERROR: `#[auto_decode]` is deprecated
-struct A;
-
-fn main() {}