Convert the `$post` parameter of `wp_get_post_parent_id()` to optional, defaulting to the current global post object when called within the loop.
Props danielpost, davidbaumwald, SergeyBiryukov, birgire, audrasjb, hellofromTonya, TimothyBlynJacobs.
Fixes#48358.
git-svn-id: https://develop.svn.wordpress.org/trunk@52194 602fd350-edb4-49c9-b593-d223f7449a82