async fn methods and impl '_ elision do not interact correctly

[nikomatsakis]

As reported by @cramertj, this example:

#![feature(async_await)]

struct Foo<'a>(&'a u8);

impl Foo<'_> {
    async fn bar() {}
}

currently produces this error:

error[E0261]: use of undeclared lifetime name `'_`

error: aborting due to previous error

but it should work just fine.