Add warn(unreachable_pub) to rustc_ast_lowering.

compiler/rustc_ast_lowering/src/lib.rs

@@ -37,6 +37,7 @@
 #![feature(box_patterns)]
 #![feature(let_chains)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::collections::hash_map::Entry;