Only generic `impl` blocks get formatted in macros

Ideally, both should be formatted.

Test:

```shell
> echo "m!(impl One {}); m!(impl<> Two {});" | rustup run nightly rustfmt
m!(impl One {});
m!(
    impl Two {}
);

> rustup run nightly rustfmt -Vv
rustfmt 1.7.0-nightly (88189a71 2024-01-19)
```