unused-lifetimes lint false-positives with async functions

Async functions that take references seem to always trigger the unused-lifetimes lint (which is allow-by-default). Example:

#![feature(async_await)]

#![warn(unused_lifetimes)]

pub async fn print(s: &str) {
    println!("{}", s);
}

warning: lifetime parameter `'_` never used
 --> src/lib.rs:5:23
  |
5 | pub async fn print(s: &str) {
  |                       ^
  |

I found this clippy issue which describes basically the same problem.

Seeing as this lint is not enabled by default, marking as deferred, though this would be good to fix and probably not too hard.

Auto merge of #4266 - uHOOCCOOHu:fix/async_fn_lifetime, r=flip1995

@rustbot

Looks the same issue as #64493.

A little inelegant but if

    !"5 | pub async fn print(s: &str) {".contains("'_")

we can suppress the lint I think...

Rollup merge of rust-lang#64603 - gilescope:unused-lifetime-warning, …

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