Fix doc cfg on reexports
Fixes#83428.
The problem was that the newly inlined item cfg propagation was not working since its real parent is different than its current one.
For the implementation, I decided to put it directly into `CfgPropagation` instead of inside `inline.rs` because I thought it would be simpler to maintain and to not forget if new kind of items are added if it's all done in one place.
r? `@notriddle`
7cffb4ca63