Allow spaces after the `.` operator if followed by a comment.

Summary: Currently, `'foo'.    // Some comment` is not allowed by the `ArcanistXHPASTLinter::LINT_BINARY_EXPRESSION_SPACING` rule. I believe that in the case of a trailing comment, we //should// allow whitespace after the `.` operator.

Test Plan: Wrote and executed a unit test for this case.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9438