Implement derives for Reborrow and CoerceShared

[P8L1]

This adds built-in derive support for the experimental Reborrow and CoerceShared marker traits behind #![feature(reborrow)].

The derives generate the same marker impls users would write manually, while preserving the existing coherence and structural validation path for the experimental reborrow traits.

Please note this impl:

• Preserves generics, lifetimes, const params, and where clauses in generated impls.
• Preserves multiple lifetime parameters in generated impls instead of duplicating the current experimental one-lifetime limitation in the derive layer.
• Requires an explicit CoerceShared target type through the derive helper attribute.
• Adds UI coverage for valid derives, feature gating, malformed helper attributes, unsupported item kinds, validation failures, and multiple-lifetime derive behavior.
• Makes the derive layer future-proof for multiple lifetimes: it preserves all lifetime parameters and generates the same impl shape as a manual impl.

Part of #145612
Part of rust-lang/rust-project-goals#399

[rustbot]

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

cc @rust-lang/rust-analyzer

[rustbot]

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[ChayimFriedman2]

Please separate the rust-analyzer changes to a new PR in the rust-analyzer repo, there is no reason for them to be here.

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[rustbot]

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

[P8L1]

@rustbot ready