Fix #[expect(dead_code)] liveness propagation

[mu001999]

Fixes #154324
Fixes #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.

[rustbot]

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