Update what seems like an outdated comment
Update what seems like an outdated comment

Totally minor, but I was taking a look at the source for educational purposes and noticed that it seems ASTNode evolved to be a union of a few additional types without the file comment being updated.   Updated to reflect the code, if I understand it correctly.
glessard/swift,apple/swift,ahoppen/swift,JGiola/swift,benlangmuir/swift,rudkx/swift,apple/swift,roambotics/swift,gregomni/swift,ahoppen/swift,apple/swift,roambotics/swift,apple/swift,gregomni/swift,roambotics/swift,rudkx/swift,ahoppen/swift,JGiola/swift,gregomni/swift,rudkx/swift,atrick/swift,glessard/swift,ahoppen/swift,ahoppen/swift,atrick/swift,roambotics/swift,gregomni/swift,glessard/swift,roambotics/swift,glessard/swift,gregomni/swift,rudkx/swift,gregomni/swift,atrick/swift,roambotics/swift,atrick/swift,JGiola/swift,JGiola/swift,benlangmuir/swift,atrick/swift,benlangmuir/swift,glessard/swift,rudkx/swift,benlangmuir/swift,ahoppen/swift,atrick/swift,apple/swift,rudkx/swift,glessard/swift,benlangmuir/swift,benlangmuir/swift,JGiola/swift,JGiola/swift,apple/swift