Consider "msort()" on "PhutilSortVector" objects to be an error

Summary:
Ref T13303. If someone upgrades from "msort()" to "msortv()" to get sort stability, but forgets to type a "v", the code executes but likely does not do what this hypothetical person intended.

Consider this an error, since I can come up with no conceivable reason to "msort()" using a method which returns "PhutilSortVector": this means you've done the legwork to stabilize the sort but are choosing unstable behavior, which makes little sense.

Test Plan: See next change.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13303

Differential Revision: https://secure.phabricator.com/D20582