WP_Query->get_posts: cache updated also when query is set to be not cacheable

Trac ticket: https://core.trac.wordpress.org/ticket/58599

Note from Abha: This issue may become merged with other performance related issues as performance team are triaging and clustering tickets. 
 
Keyword: needs-dev-note
Ticket owner: spacedmonkey
Core committer: spacedmonkey