Extend LINT_BRACE_FORMATTING to warn on missing braces

Summary: Extend the ArcanistXHPASTLinter::LINT_BRACE_FORMATTING rule to raise a warning when n_STATEMENT is used without having n_STATEMENT_LIST as a parent. Essentially, this means that if ($x) { do_y(); } is preferred over if ($x) do_y();.

Test Plan: Added some test cases.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10571