Auto merge of #127097 - compiler-errors:async-closure-lint, r=oli-obk

Implement simple, unstable lint to suggest turning closure-of-async-block into async-closure

We want to eventually suggest people to turn `|| async {}` to `async || {}`. This begins doing that. It's a pretty rudimentary lint, but I wanted to get something down so I wouldn't lose the code.

Tracking:
* #62290

Author: bors