Lint adjacent borrowed-slice temps in unnecessary_to_owned

[1mpossible-code]

Closes #16779

• lint cases where to_vec() / to_owned() is saved into an immutable local and then right away borrowed again as a slice in the next statement or tail expression
• keep the new logic pretty narrow by only looking at single use adjacent locals with simple borrowed receivers and skipping mutable bindings and loop or multi-call closure cases
• add UI tests for the reported pattern, the tail-expression form, and negative cases like non-adjacent use, mutation, multiple uses, loops, closures, and earlier false-positive variants

Open to feedback or adjustments if this should be more/less strict

changelog: [unnecessary_to_owned]: lint adjacent local slice reborrows from to_vec() / to_owned()

[rustbot]

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