more clippy fixes: clippy::{iter_cloned_collect, unwarp_or_else_default, option_map_or_none}

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -1317,7 +1317,7 @@ fn compare_number_of_generics<'tcx>(
                         impl_count,
                         kind,
                         pluralize!(impl_count),
-                        suffix.unwrap_or_else(String::new),
+                        suffix.unwrap_or_default(),
                     ),
                 );
             }