Update grammar for new fn grammar

Update for https://github.com/rust-lang/rust/pull/68764 and https://github.com/rust-lang/rust/pull/68788 and https://github.com/rust-lang/rust/pull/69023 and https://github.com/rust-lang/rust/pull/68728