Add warn(unreachable_pub) to rustc_llvm.

compiler/rustc_llvm/src/lib.rs

@@ -3,6 +3,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 // NOTE: This crate only exists to allow linking on mingw targets.