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