Users: Make sure `wp_logout()` clears current user, so that a subsequent call...

Users: Make sure `wp_logout()` clears current user, so that a subsequent call to `is_user_logged_in()` works as expected.

Props sebastian.pisula, donmhico, roytanck, juliobox.
Fixes #35488.
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git-svn-id: https://core.svn.wordpress.org/trunk@46265 1a063a9b-81f0-0310-95a4-ce76da25c4cd