Ignore nightly's new elided_named_lifetimes lint in generated code

[dtolnay]

rust-lang/rust#129207 added this lint:

warning: elided lifetime has a name
    --> tests/test.rs:1118:30
     |
1118 |         async fn f(&self) -> &str
     |                    -         ^ this elided lifetime gets resolved as `'life0`
     |                    |
     |                    lifetime `'life0` declared here
     |
     = note: `#[warn(elided_named_lifetimes)]` on by default

For #[async_trait] this is not useful because the macro needs to give names to some lifetimes which are not named in the user's code.