Add warn(unreachable_pub) to rustc_lexer.

compiler/rustc_lexer/src/lib.rs

@@ -23,6 +23,7 @@
 // We want to be able to build this crate with a stable compiler,
 // so no `#![feature]` attributes should be added.
 #![deny(unstable_features)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 mod cursor;