Added PhantomPinned diagnostic item and prevented dead field warning on PhantomPinned

[asder8215]

This PR closes #154888. See #154978 and in clippy#17056 for prior history on working on this issue.

From discussing with clippy team, we thought it'd be appropriate to use a diagnostic item attribute to mark PhantomPinned and prevent dead field warnings from being emitted in missing_fields_in_debug.rs and pub_underscore_fields.rs.

If you want me to do the clippy changes separately within the clippy repository, I can do that.

[rustbot]

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

[rustbot]

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[BoxyUwU]

lgtm including the clippy bits, i assume clippy folks will want to review this though, so r=me if clippy is fine with this :3

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[BoxyUwU]

r? clippy

[rustbot]

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

[ada4a]

Also, in my experience, when needing to add a diagnostic item for clippy, one usually makes a PR to rustc adding the diagnostic item, waits for a sync, and then does the remaining changes as a PR to clippy. Not sure how strongly this rule is followed though – I haven't been around for that long^^

[asder8215]

Also, in my experience, when needing to add a diagnostic item for clippy, one usually makes a PR to rustc adding the diagnostic item, waits for a sync, and then does the remaining changes as a PR to clippy. Not sure how strongly this rule is followed though – I haven't been around for that long^^

Yeah, I'm aware that changes to clippy is separated from PRs made to the rust repo. But I also figured that this change was small enough and quicker to do atomically here.