autocomplete breaks in an item identity macro.

consider the following macro:

macro_rules! item_ident {
    ($($item:item)*) => {
        $( $item )*
    }
}

When writing code within this macro, syntax as we're typing might not be considered a valid "item". for example:

item_ident! {
    fn baz() {
        let x = <|>
    }
}

Of course rust-analyzer complains about this:

expected Item
rust-analyzer[macro-error](https://rust-analyzer.github.io/manual.html#macro-error)

This is a fairly common pattern, for example, used in tokio to wrap code blocks with valid #[cfg(...)]s: https://github.com/tokio-rs/tokio/blob/master/tokio-util%2Fsrc%2Fcfg.rs

I wonder if we can recover from these syntax errors such that autocomplete and the likes work in this situation.

Reproduce the issue with the item_ident! macro shown in the report and compare behavior with the Tokio tokio-util/src/cfg.rs example. Investigate rust-analyzer's macro parsing and recovery around an incomplete expression such as <|>. Done means autocomplete and related IDE features work inside the macro despite the temporary item syntax error.