When assigning to them, we can't classify the expression type, because
that doesn't contain the right information.
And when reading from them, we need to do the extra deref, just like we
do when reading from a DeclRefExpr.
Differential Revision: https://reviews.llvm.org/D136012
References are implemented through pointers, so we need a second deref
when encountering a DeclRefExpr of a reference type.
Differential Revision: https://reviews.llvm.org/D132997