Add warn(unreachable_pub) to rustc_passes.

compiler/rustc_passes/src/lib.rs

@@ -12,6 +12,7 @@
 #![feature(map_try_insert)]
 #![feature(rustdoc_internals)]
 #![feature(try_blocks)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use rustc_middle::query::Providers;