C++: fix crash during code completion with base template class

ResolveExpression has to have a reference of ContextLookup. If not there
will be a crash because of deleted instanitated base template class.

Task-number: QTCREATORBUG-9329
Change-Id: I7f8c83da0d81ac6311e76d15a897adbc70b08d75
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>