Improve node upmapping into macro inputs

[Veykril]

Noticed in #16229

Taking async-trait as an example, given:

#[async_trait]
trait Advertisement {
    async fn run(&self);
}

fn f<T: Advertisement>(t: T) {
    t.run()
}

and the expansion of the attribute as:

trait Advertisement {
    #[must_use]
    #[allow(clippy::type_complexity, clippy::type_repetition_in_bounds)]
    fn run<'life0, 'async_trait>(
        &'life0 self,
    ) -> ::core::pin::Pin<
        Box<dyn ::core::future::Future<Output = ()> + ::core::marker::Send + 'async_trait>,
    >
    where
        'life0: 'async_trait,
        Self: 'async_trait;
}

When we are trying to upmap the run function node from the expansion, we will fail due to all the new tokens that have been added that have varying spans and more importantly a differing syntax context from the input name (differing syntax context is an immediate bail currently). Ideally we'd still map the signature out of this, the question is how.

[JakobDegen]

@Veykril any idea if changes to async_trait can bypass the issues here?

[Veykril]

The attribute could accommodate for us here but that would be the wrong way to go about this. The attribute is written with good diagnostic output in mind (that's where spans are mainly relevant). We should be able to improve this on our side, it's just that the logic for that be fleshed out a bit as currently its just very simplistic. That is figure out which tokens we can ignore while upmapping, which ones are relevant etc