Use of `.await` in 2015 edition prints one error per use

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When using .await and async fn in 2015 editions, we emit an error for each instance of incorrect usage. The most common case of this happening is likely to be when not being aware of the edition distinction in a new project and/or copying complete code from elsewhere. We should group these errors and emit a single error to reduce output verbosity. We might want to group per mod or some other strategy, but even the naïve approach is likely to work out ok.