Add a linter rule to detect duplicate case statements

Summary: Fixes T6843. Adds a linter rule to ArcanistXHPASTLinter to detect duplicate case statements within a switch statement.

Test Plan: Added test cases.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T6843

Differential Revision: https://secure.phabricator.com/D11171