Prior to the introduction of `WP_Term_Query`, the 'orderby' clause
passed to the 'terms_clauses' filter was prefixed by `ORDER BY`. After
`WP_Term_Query`, this was not the case; `ORDER BY` was added after the
filter. As such, plugins filtering 'terms_clauses' and returning an
'orderby' clause beginning with `ORDER BY` resulted in invalid syntax
when `WP_Term_Query` prepended a second `ORDER BY` keyword to
the clause.
This changeset rearranges the way the 'orderby' clause is built so that
it will be passed to 'terms_clauses' in the previous format.
Fixes#37378.
git-svn-id: https://develop.svn.wordpress.org/trunk@38099 602fd350-edb4-49c9-b593-d223f7449a82
103da159d5