C++: Handle C++ style comments in macro expansion

Notice that a similar problem still exists for which we
need to fix the lexer when there's a C style commend which
 ends with a backslash-newline.

Task-number: QTCREATORBUG-7713

Change-Id: I0f6d561703984f917fa5ed29de020ad0bdc5aaf0
Reviewed-by: David Schulz <david.schulz@nokia.com>
Reviewed-by: hjk <qthjk@ovi.com>