Add warn(unreachable_pub) to rustc_passes.

compiler/rustc_passes/src/lang_items.rs

@@ -359,6 +359,6 @@ impl<'ast, 'tcx> visit::Visitor<'ast> for LanguageItemCollector<'ast, 'tcx> {
     }
 }
 
-pub fn provide(providers: &mut Providers) {
+pub(crate) fn provide(providers: &mut Providers) {
     providers.get_lang_items = get_lang_items;
 }