Add warn(unreachable_pub) to rustc_span.

compiler/rustc_span/src/lib.rs

@@ -30,6 +30,7 @@
 #![feature(round_char_boundary)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 // The code produced by the `Encodable`/`Decodable` derive macros refer to