rustc: Add lint for obsolete attributes

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

src/libsyntax/lib.rs

@@ -83,7 +83,6 @@ pub mod ext {
     pub mod concat;
     pub mod concat_idents;
     pub mod log_syntax;
-    pub mod auto_encode;
     pub mod source_util;
 
     pub mod trace_macros;