Implemented phutil_utf8_is_combining_character().
Summary: Implemented phutil_utf8_is_combining_character(), which returns true if an input character is a combining character.
Test Plan: Unit test: testUTF8IsCombiningCharacter().
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin, AnhNhan
Maniphest Tasks: T2379
Differential Revision: https://secure.phabricator.com/D5981