[SyntaxTree] Provide `List::classof`

clang/include/clang/Tooling/Syntax/Tree.h

public:
 
   enum class TerminationKind {
     Terminated,
     MaybeTerminated,
     Separated,
   };
 
   using Tree::Tree;
+  static bool classof(const Node *N);
   /// Returns the elements and corresponding delimiters. Missing elements
   /// and delimiters are represented as null pointers.
   ///
   /// For example, in a separated list:
   /// "a, b, c" <=> [("a", ","), ("b", ","), ("c", null)]
   /// "a, , c" <=> [("a", ","), (null, ","), ("c", ",)]
   /// "a, b," <=> [("a", ","), ("b", ","), (null, null)]
   ///

clang/lib/Tooling/Syntax/Tree.cpp

 syntax::Node *syntax::Tree::findChild(NodeRole R) {
   for (auto *C = FirstChild; C; C = C->getNextSibling()) {
     if (C->getRole() == R)
       return C;
   }
   return nullptr;
 }
 
+bool classof(const syntax::Node *N) {
+  switch (N->getKind()) {
+  case syntax::NodeKind::NestedNameSpecifier:
+  case syntax::NodeKind::CallArguments:
+  case syntax::NodeKind::ParameterDeclarationList:
+    return true;
+  default:
+    return false;
+  }
+}
+
 std::vector<syntax::List::ElementAndDelimiter<syntax::Node>>
 syntax::List::getElementsAsNodesAndDelimiters() {
   if (!getFirstChild())
     return {};
 
   auto children = std::vector<syntax::List::ElementAndDelimiter<Node>>();
   syntax::Node *elementWithoutDelimiter = nullptr;
   for (auto *C = getFirstChild(); C; C = C->getNextSibling()) {