Use impl Iterator in arg position in clippy_dev

Small refactoring pulled out of work on #3266.
This should make the methods a bit more flexible.

clippy_dev/src/main.rs

@@ -51,5 +51,5 @@ fn print_lints() {
         }
     }
 
-    println!("there are {} lints", Lint::active_lints(&lint_list).count());
+    println!("there are {} lints", Lint::active_lints(lint_list.into_iter()).count());
 }