Add warn(unreachable_pub) to rustc_driver_impl.

Author: nnethercote

compiler/rustc_driver_impl/src/lib.rs

@@ -15,6 +15,7 @@
 #![feature(panic_update_hook)]
 #![feature(result_flattening)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::cmp::max;