C++: do not re-highlight if the document in the semantic info is invalid

Change-Id: I12561ae0dc0e1912b3aefde92deff7b8da84e3be
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
C++: remove superfluous forward defines.

Change-Id: I962e0fbafdb2281645b98779a421d1084d223570
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>