Add warn(unreachable_pub) to rustc_hir.

Author: nnethercote

compiler/rustc_hir/src/lib.rs

@@ -10,6 +10,7 @@
 #![feature(never_type)]
 #![feature(rustc_attrs)]
 #![feature(variant_count)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 extern crate self as rustc_hir;