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