Add warn(unreachable_pub) to several crates.

It requires no additonal changes to these crates, but will prevent
unnecessary `pub`s in the future.

compiler/rustc_abi/src/lib.rs

@@ -3,6 +3,7 @@
 #![cfg_attr(feature = "nightly", doc(rust_logo))]
 #![cfg_attr(feature = "nightly", feature(rustdoc_internals))]
 #![cfg_attr(feature = "nightly", feature(step_trait))]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::fmt;

compiler/rustc_ast/src/lib.rs

@@ -19,6 +19,7 @@
 #![feature(never_type)]
 #![feature(rustdoc_internals)]
 #![feature(stmt_expr_attributes)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub mod util {

compiler/rustc_ast_ir/src/lib.rs

@@ -2,6 +2,7 @@
 #![cfg_attr(feature = "nightly", allow(internal_features))]
 #![cfg_attr(feature = "nightly", feature(never_type))]
 #![cfg_attr(feature = "nightly", feature(rustc_attrs))]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 #[cfg(feature = "nightly")]