Implement coercions between &pin (mut|const) T and &(mut) T when T: Unpin

[frank-king]

This allows the following (mutual) coercions when T: Unpin:

• &T <-> Pin<&T>
• &mut T <-> Pin<&mut T>
• &mut T -> Pin<&T>
• Pin<&mut T> -> &T

Part of Pin Ergonomics.

[rustbot]

Some changes occurred in need_type_info.rs

cc @lcnr

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred in exhaustiveness checking

cc @Nadrieril

[rustbot]

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[nnethercote]

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[lcnr]

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[spastorino]

r? types

[bors]

☔ The latest upstream changes (presumably #148602) made this pull request unmergeable. Please resolve the merge conflicts.

[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.

[bors]

☔ The latest upstream changes (presumably #150115) made this pull request unmergeable. Please resolve the merge conflicts.