Fix the desugaring of dependent decltype and typeof(expr) nodes. The

isSugared() and desugar() routines previously provided were never
actually called, since the corresponding types
(DependentTypeOfExprType, DependentDecltypeType) don't have
corresponding type classes. Outside of the current (incomplete) patch
I'm working on, I haven't found a way to trigger this problem.

llvm-svn: 134973