Port #[coverage] to the new attribute system

compiler/rustc_parse/src/validate_attr.rs

@@ -318,6 +318,7 @@ pub fn check_builtin_meta_item(
                 | sym::rustc_layout_scalar_valid_range_end
                 | sym::no_implicit_prelude
                 | sym::automatically_derived
+                | sym::coverage
         ) {
             return;
         }