async fn should be disallowed entirely on 2015 edition

[nikomatsakis]

As described in #53714, we currently permit a small amount of async notation in 2015, but we can't extend that to the complete feature set (e.g., await keywords etc). This was done as part of a transitionary tactic (so people could move to Rust 2018) but by now that is not necessary. We should just prohibit async fn in Rust 2015 entirely at this point.

[nikomatsakis]

Note we should also test nested async fn in fns, as described in #51933

[cofibrant]

Hello, I'd like to have a go at this