Improve do_enclose() logic on post publish.
Removing the direct SQL query in do_all_pings() improves filterability.
As part of this change, the signature of do_enclose() is changed so that
a null $content parameter can be passed, with the $content then inferred
from the $post passed in the second parameter. In addition, the second
parameter was modified so that a post ID or a WP_Post object can be
provided. These changes make it possible to trigger enclosure checks with
a post ID alone (as in do_all_pings()) and also brings the function
signature in line with do_trackbacks() and pingback().
Props dshanske, spacedmonkey, janw.oostendorp, mrmadhat, birgire.
See #36824.