Maybe it's xorg and not xfce. Why would an OS upgraded destroy one of it's installed components?! That is my point. I have not been able to fix this issue. I did research a couple of time but it struck me as a waste of time to have to do that in the first place.Are you sure that it is Xfce that is getting hosed and not some of Xorg's components?
pkg info | grep <partial portname>
to see if your applications are still there. From here, you should be able to figure it out.root@kg-quiet# pkg info llvm*
llvm39-3.9.1_6
llvm40-4.0.1