Fix `#[expect(dead_code)]` liveness propagation
Fixes https://github.com/rust-lang/rust/issues/154324
Fixes https://github.com/rust-lang/rust/issues/152370 (cc @eggyal)
Previously, when traversing from a `ComesFromAllowExpect::Yes` item (i.e., with `#[allow(dead_code)]` or `#[expect(dead_code)]`), other `ComesFromAllowExpect::Yes` items reached during propagation would be updated to `ComesFromAllowExpect::No` and inserted into `live_symbols`. That caused `dead_code` lint couldn't be emitted correctly.
After this PR, `ComesFromAllowExpect::Yes` items no longer incorrectly update other `ComesFromAllowExpect::Yes` items during propagation or mark them live by mistake, then `dead_code` lint could behave as expected.
Jacob Pratt
3b4c17915e