Query: Make sure the `found_posts` property of `WP_Query` is always an...

Query: Make sure the `found_posts` property of `WP_Query` is always an integer, to match the documented type.

This makes the property consistent with similar properties of other classes:
* `WP_Comment_Query::$found_comments`
* `WP_Network_Query::$found_networks`
* `WP_Site_Query::$found_sites`
* `WP_User_Query::$total_users`

Props birgire, PressLabs.
Fixes #42469.
Built from https://develop.svn.wordpress.org/trunk@48328


git-svn-id: https://core.svn.wordpress.org/trunk@48097 1a063a9b-81f0-0310-95a4-ce76da25c4cd