There are three cases that must be handled:

 1. Completion in C++ style comment
 2. Completion in first line of a C style comment
 3. Completion in non-first line of a C style comment

This change fixes case 1 + 2. Case 3 will be addressed in a follow-up
change, same goes for the duplication.

Task-number: QTCREATORBUG-15143
Change-Id: I449711f965ddcbbe6158870a8a5ae33218e0d238
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>