Ignore generated fresh lifetimes in elision check
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fixes#3988
changelog: Ignore generated fresh lifetimes in elision check.
**HELP**: It seems `tests/ui` are compiled under edition 2015, and I don't know how to add tests for this properly.
Here is the test input it had already passed:
```rust
#![feature(async_await)]
#![allow(dead_code)]
async fn sink1<'a>(_: &'a str) {} // lint
async fn sink1_elided(_: &str) {} // ok
async fn one_to_one<'a>(s: &'a str) -> &'a str { s } // lint
async fn one_to_one_elided(s: &str) -> &str { s } // ok
async fn all_to_one<'a>(a: &'a str, _b: &'a str) -> &'a str { a } // ok
// async fn unrelated(_: &str, _: &str) {} // Not allowed in async fn
// #3988
struct Foo;
impl Foo {
pub async fn foo(&mut self) {} // ok
}
// rust-lang/rust#61115
async fn print(s: &str) { // ok
println!("{}", s);
}
fn main() {}
```
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