Rollup merge of #120402 - compiler-errors:async-closure-def-tree, r=cjgillot

Make the coroutine def id of an async closure the child of the closure def id

Adjust def collection to make the (inner) coroutine returned by an async closure be a def id child of the (outer) closure. This makes it easy to map from coroutine -> closure by using `tcx.parent`, since currently it's not trivial to do this.

Author: GuillaumeGomez