[MS Demangler] Add virtual destructor.

Silence -Wnon-virtual-dtor.

llvm-svn: 340711
diff --git a/llvm/lib/Demangle/MicrosoftDemangleNodes.h b/llvm/lib/Demangle/MicrosoftDemangleNodes.h
index 4fdb059..df4679e 100644
--- a/llvm/lib/Demangle/MicrosoftDemangleNodes.h
+++ b/llvm/lib/Demangle/MicrosoftDemangleNodes.h
@@ -324,6 +324,7 @@
 
 struct Node {
   explicit Node(NodeKind K) : Kind(K) {}
+  virtual ~Node() = default;
 
   NodeKind kind() const { return Kind; }