Add warn(unreachable_pub) to rustc_expand.

Plus a tiny bit of reformatting.

compiler/rustc_expand/src/lib.rs

@@ -13,6 +13,7 @@
 #![feature(rustdoc_internals)]
 #![feature(try_blocks)]
 #![feature(yeet_expr)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 extern crate proc_macro as pm;