Respect `#[expect]` the same way `#[allow]` is with the `dead_code` lint
This PR makes the `#[expect]` attribute being respected in the same way the `#[allow]` attribute is with the `dead_code` lint.
The fix is much more involved than I would have liked (and it's not because I didn't tried!), because the implementation took advantage of the fact that firing a lint in a allow context is a nop (for the user, as the lint is suppressed) to not fire-it at all.
And will it's fine for `#[allow]`, it definitively isn't for `#[expect]`, as the presence and absence of the lint is significant. So a big part of the PR is just adding the context information of whenever an item is on the worklist because of an `[allow]`/`#[expect]` or not.
Fixes https://github.com/rust-lang/rust/issues/114557
1e836d12d3