Support underscore as associated const name

[makai410]

This PR doesn't change the current behavior of the dead_code lint for unused associated consts with underscore-prefixed names and treats associated consts named _ consistently.

There might still be some places that should've been changed to use named_items() instead of in_definition_order(), though I'm not quite sure.

Unresolved questions:

• Whether to lint impl Private { pub const _: Private = Private; }, where the visibility qualifier here has no effect. It seems like there wasn't any disagreement on linting this, though what's proposed in the RFC currently diverges from that: RFC: Associated const underscore.

Tracking:

• Tracking issue for RFC 3527: Associated const underscore #158944

[rustbot]

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a

HIR ty lowering was modified

cc @fmease

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

cc @rust-lang/clippy

These commits modify tests/rustdoc-json.
rustdoc-json is a public (but unstable) interface.

Please ensure that if you've changed the output:

• It's intentional.
• The FORMAT_VERSION in src/librustdoc-json-types is bumped if necessary.

cc @obi1kenobi

[rustbot]

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.