There are two versions of startOfOperator:
    * InternalCppCompletionAssistProcessor::startOfOperator
    * ClangCompletionAssistProcessor::startOfOperator

The latter started as a copy of the former, but the former got some bug
fixes in the meantime.  Adjust both versions to each other, so it's easy
to diff them and to extract the duplication in a follow-up change.

Change-Id: Icf48386bf1ad0fa473bec476c5412be9b1890139
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>