C++: fix multi-line continuation handling.

When having doxygen enabled, but leading astriskses disabled, a comment
continuation line did not insert an extra space for where the astrisks
would have been, thus mis-aligning the second line.

Task-number: QTCREATORBUG-12539
Change-Id: I47baa203fd5d9279fa89a06f526f3abed06ce443
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>