Add warn(unreachable_pub) to rustc_symbol_mangling.

Author: nnethercote

compiler/rustc_symbol_mangling/src/lib.rs

@@ -93,6 +93,7 @@
 #![doc(rust_logo)]
 #![feature(let_chains)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use rustc_hir::def::DefKind;