Use the primary `meta_query` clause when parsing `orderby` in `WP_Query`.

When using legacy `meta_key`, `meta_value`, etc. arguments in `WP_Query`,
they're converted into the first clause of a `meta_query`. By using that
clause instead of the original arguments, we make sure that behavior is
consistent between the two available formats.

props boonebgorges.
fixes #16814.


git-svn-id: https://develop.svn.wordpress.org/trunk@29855 602fd350-edb4-49c9-b593-d223f7449a82