REST API: Add support for filename search in media endpoint.

In [38625], the functionality to search for attachments by filename was added
via the `posts_clauses` filter and the `_filter_query_attachment_filenames()`
function.  This moves `_filter_query_attachment_filenames()` from
`wp-admin/includes/post.php` to `wp-includes/post.php` so that it can be
applied in the same manner in the REST API media endpoint.

Props jblz, tyxla.
Fixes #39092.

Built from https://develop.svn.wordpress.org/trunk@39598


git-svn-id: https://core.svn.wordpress.org/trunk@39538 1a063a9b-81f0-0310-95a4-ce76da25c4cd