C++: Support __thread and thread_local

Task-number: QTCREATORBUG-7679
Change-Id: I794f52b2bcfb6c78ceef86ec53b6ed32b3d53d9f
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>