avoid Symbol to &str conversions

compiler/rustc_resolve/src/diagnostics.rs

@@ -1630,7 +1630,7 @@ impl<'a> Resolver<'a> {
                     "{}{} `{}` defined here",
                     prefix,
                     suggestion.res.descr(),
-                    suggestion.candidate.as_str(),
+                    suggestion.candidate,
                 ),
             );
         }