qmake doesn't process TreatSpecificWarningsAsErrors anymore

In 4.7 there was some code to process TreatSpecificWarningsAsErrors when generating VS project files (though it had a bug as it used = instead of +=, so only one value was kept):
http://qt.gitorious.org/qt/qt/blobs/4.7/qmake/generators/win32/msbuild_objectmodel.cpp#line1045

This code was removed since there was a lot of redundancy (see msvc_objectmodel). But msvc_objectmodel doesn't handle this anymore. Maybe there is even more stuff like this that was in msbuild_objectmodel and got lost.