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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[mu001999]

@rustbot author

I realized that this would fail during secondary propagation. This still needs to be revised a bit.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[mu001999]

oops, forgot to update the label

@rustbot ready

[nikomatsakis]

View changes since the review

Nit: I found this struct hard to understand, I think a doc comment that explains the role of these fields in terms of examples would be very helpful to future code readers :)

[mu001999]

Added, could you review this again? :)

[joshtriplett]

See #152370 (comment) .

[joshtriplett]

Test cases look good here.