With https://jira.mongodb.org/browse/SERVER-70871, we will now go through query planning (call `QueryPlanner::plan`) twice in cases where a query is eligible for SBE but the plan is not (i.e., `attemptToGetSlotBasedExecutor` builds the query plan but then still falls back to the classic engine). We should look into whether QuerySolutions can be shared/passed to the `getClassicExecutor` path in this case. 

Note that it may be difficult or not possible to do this at times, as some SBE plans might not be valid for classic execution (e.g., if they use `COLUMN_SCAN`).