This changeset ports a number of 'orderby' features from `WP_Query` and
`WP_Comment_Query`:
* Allow multiple 'orderby' values to be passed as a space-separated list.
* Allow multiple 'orderby' values to be passed as a flat array.
* Allow multi-dimensional 'orderby', with orderby fields as array keys and ASC/DESC as the corresponding values.
See #31265.
git-svn-id: https://develop.svn.wordpress.org/trunk@31663 602fd350-edb4-49c9-b593-d223f7449a82
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