Fix diagnostics for coroutines with () as input.

[senioria]

This may be a more real-life example to trigger the diagnostic:

#![features(try_blocks, coroutine_trait, coroutines)]

use std::ops::Coroutine;

struct Request;
struct Response;
fn get_args() -> Result<String, String> { todo!() }
fn build_request(_arg: String) -> Request { todo!() }
fn work() -> impl Coroutine<Option<Response>, Yield = Request> {
    #[coroutine]
    |_| {
        let r: Result<(), String> = try {
            let req = get_args()?;
            yield build_request(req)
        };
        if let Err(msg) = r {
            eprintln!("Error: {msg}");
        }
    }
}

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @compiler-errors (or someone else) some time within the next two weeks.

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[compiler-errors]

@bors r+ rollup

[bors]

📌 Commit 986e20d has been approved by compiler-errors

It is now in the queue for this repository.