Improve autofixing of double quotes in XHPAST linter.

Summary: The XHPAST double quotes rule autofixes strings which use double quotes where single quotes would suffice. In the case in which the double-quoted string contained an escaped double quote, the autofix string can be improved by removing the escape symbol.

Test Plan: Modified existing unit tests to cover this case.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9196