Returning a non-null value from the new `posts_pre_query` filter will cause
`WP_Query` to skip its database query, so that posts data can be provided from
elsewhere. This is useful in cases where post data may be mirrored in a
separate location, such as an external search application.
Developers should note that the `WP_Query` properties generally used to
calculate pagination - specifically, `found_posts` and `max_num_pages`, which
are determined by default in `set_found_posts()` - must be provided explicitly
when using the `posts_pre_query` filter; since `WP_Query` will not be
contacting the database, it will have no access to `SELECT FOUND_ROWS()`.
The `WP_Query` instance is passed to `posts_pre_query` by reference, so that
these properties can be set manually if needed.
Props jpdavoutian, tlovett1.
Fixes#36687.
git-svn-id: https://develop.svn.wordpress.org/trunk@37692 602fd350-edb4-49c9-b593-d223f7449a82
becae4f492