qdoc: Fixed a regression bug caused by fixing error messages

qdoc: Fixed a regression bug caused by fixing error messages

The C++ code marker is the default code marker. The default
code marker was being called for .qdoc files. But when the
tree nodes were each assigned a location object based on
the location in the source file where the node was built,
the default code marker was no longer used. Instead, the
plain code marker was used. This was wrong. qdoc now knows
to use the C++ code marker for all .qdoc files.

Change-Id: I15a58168db74cc5aa82a1fbccc5b7ece219ec297
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>