expand: Change how `#![cfg(FALSE)]` behaves on crate root
Previously it removed all other attributes from the crate root.
Now it removes only attributes below itself (during both regular expansion and pre-configuration).
So it becomes possible to configure some global crate properties even for fully unconfigured crates.
Fixes https://github.com/rust-lang/rust/issues/104633
Part of https://github.com/rust-lang/rust/issues/110082