Allow "PhutilClassMapQuery" to sort with a PhutilSortVector sort function

Summary:
Ref T13303. I didn't find any explicit "msortv()" issues in instances/, but did catch one during deploy where we run a ClassMapQuery with a vector-returning sort method.

Test if the sort method returns vectors and "msortv()" it if it does.

Test Plan: Will cherry-pick and redeploy, just throwing this up for reference/context.

Reviewers: amckinley

Maniphest Tasks: T13303

Differential Revision: https://secure.phabricator.com/D20611