Further improvements to keyword casing linter rule

Summary: Ref T7409. Improve the ArcanistXHPASTLinter::LINT_KEYWORD_CASING linter rule to check that magic constants are uppercase.

Test Plan: Added unit test.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7409

Differential Revision: https://secure.phabricator.com/D12368