Date/Time: Make sure `get_post_time()` keeps UTC time on timezone change.

Add `$source` parameter to `get_post_datetime()` to instantiate from local or UTC time in database.

Props Rarst, david.binda.
Reviewed by azaozz, SergeyBiryukov.
Fixes #48384.
Built from https://develop.svn.wordpress.org/trunk@46580


git-svn-id: https://core.svn.wordpress.org/trunk@46377 1a063a9b-81f0-0310-95a4-ce76da25c4cd