Resolve XHPAST linter futures on background

Summary: This is a little bit tricky - if both XHPAST and PhutilXHPAST linters lint the same path then they get the same future wrapped in two different Future iterators.

Test Plan:
  $ arc unit

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5015