refactor: remove Adjust::ReborrowPin

[frank-king]

Followed by #149130, this PR removes Adjust::ReborrowPin and use an Adjust::Deref(DerefAdjustKind::Pin) followed by an Adjust::Borrow(AutoBorrow::Pin) instead.

[rustbot]

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[frank-king]

@rustbot blocked

r? jackh726

[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.

[frank-king]

@rustbot ready