Fix handling of const_cast from prvalue to rvalue reference: such a cast is
only permitted if the source object is of class type, and should materialize a
temporary for the reference to bind to.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184017 91177308-0d34-0410-b5e6-96231b3b80d8