7d60819bfd
typeck in parallel #108118 caused `typeck` to be transferred to the serial part (`check_unused`), which made the performance of parallel rustc significantly reduced. This pr re-parallelize this part, which increases the average performance improvement of parallel rustc in `full` and `incr-full` scenarios from [14.4%](https://github.com/rust-lang/rust/pull/110284#issuecomment-1545354608) to [23.2%](https://github.com/rust-lang/rust/pull/110284#issuecomment-1624770626). r? `@cjgillot` cc `@oli-obk` `@Zoxc`
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.