ClangCodeModel: Fix re-parsing with new options

If the command line options changed unload the unit so the next
SemanticMarker::reparse() will call clang_parseTranslationUnit() with
the new options.

Task-number: QTCREATORBUG-11606
Change-Id: I44e3da97850abd875a88abde0b406b14198b5be8
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>