Revisions: Restore the return value of `wp_get_post_autosave()` to the...

Revisions: Restore the return value of `wp_get_post_autosave()` to the documented type of `WP_Post` for backward compatibility.

Follow-up to [48422].

See #34560.
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git-svn-id: https://core.svn.wordpress.org/trunk@48207 1a063a9b-81f0-0310-95a4-ce76da25c4cd