mbe: Make a missing macro_attr feature non-fatal

Parse the macro and its attr rules anyway, after emitting the error.

Author: joshtriplett

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -556,9 +556,8 @@ pub fn compile_declarative_macro(
         let args = if p.eat_keyword_noexpect(sym::attr) {
             has_attr_rules = true;
             if !features.macro_attr() {
-                let msg = "`macro_rules!` attributes are unstable";
-                let e = feature_err(sess, sym::macro_attr, span, msg);
-                return dummy_syn_ext(e.emit());
+                feature_err(sess, sym::macro_attr, span, "`macro_rules!` attributes are unstable")
+                    .emit();
             }
             if let Some(guar) = check_no_eof(sess, &p, "expected macro attr args") {
                 return dummy_syn_ext(guar);