Fixes a bug in DynTypedNode.

Two DynTypedNodes can be equal if they do not have the same node type,
because DynTypedNodes for the same underlying object might have been
created from different types (for example, Decl vs VarDecl).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204722 91177308-0d34-0410-b5e6-96231b3b80d8