Posts, Post Types: Make `get_post_states()` available on frontend.

This allows special pages to be denoted as such when editing a menu in the Customizer.

This applies to the Front Page, Posts Page, and Privacy Policy Page.

Follow-up to [47211], [47213], [47763].

Props dlh, whyisjake, SergeyBiryukov.
Fixes #46829. See #49374.
Built from https://develop.svn.wordpress.org/trunk@48619


git-svn-id: https://core.svn.wordpress.org/trunk@48381 1a063a9b-81f0-0310-95a4-ce76da25c4cd