Allow formatting macro decl with invalid syntax to fail

This change is necessary due to the recent change in the rustc parser that the
following attribute has become a hard parser error:

```rust
struct Foo;
```

cc rust-lang/rust#57367.

Author: topecongiro

tests/target/macro_rules.rs

@@ -172,7 +172,7 @@ macro_rules! m [
 ];
 
 // #2470
-macro foo($type_name:ident, $docs:expr) {
+macro foo($type_name: ident, $docs: expr) {
     #[allow(non_camel_case_types)]
     #[doc=$docs]
     #[derive(Debug, Clone, Copy)]