WP_Query: allow split_the_query = false to avoid a split.

see #26937.

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git-svn-id: http://core.svn.wordpress.org/trunk@27476 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Return a bool from wp_using_ext_object_cache(), never a null.

see #26937.

Built from https://develop.svn.wordpress.org/trunk@27634


git-svn-id: http://core.svn.wordpress.org/trunk@27477 1a063a9b-81f0-0310-95a4-ce76da25c4cd