Add warn(unreachable_pub) to rustc_target.

compiler/rustc_target/src/lib.rs

@@ -17,6 +17,7 @@
 #![feature(let_chains)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::path::{Path, PathBuf};