Add a property that returns a Syntax node as its concrete type

Otherwise it requires a lot of boilerplate to return the Syntax node's
type in the lit-test-helper.

Author: ahoppen

Sources/SwiftSyntax/SyntaxNodes.swift.gyb

@@ -314,6 +314,20 @@ public extension Syntax {
 %   else:
       return .${node.swift_syntax_kind}(${node.name}(self)!)
 %   end
+% end
+    }
+  }
+
+  /// Retrieve the concretely typed node that this Syntax node wraps.
+  var asConcreteType: Any {
+    switch self.asSyntaxEnum {
+    case .token(let node):
+      return node
+    case .unknown(let node):
+      return node
+% for node in SYNTAX_NODES:
+    case .${node.swift_syntax_kind}(let node):
+      return node
 % end
     }
   }

Sources/lit-test-helper/main.swift

@@ -399,11 +399,11 @@ func performRoundtrip(args: CommandLineArguments) throws {
 struct NodePrinter: SyntaxAnyVisitor {
   func visitAny(_ node: Syntax) -> SyntaxVisitorContinueKind {
     assert(!node.isUnknown)
-    print("<\(type(of: node))>", terminator: "")
+    print("<\(type(of: node.asConcreteType))>", terminator: "")
     return .visitChildren
   }
   func visitAnyPost(_ node: Syntax) {
-    print("</\(type(of: node))>", terminator: "")
+    print("</\(type(of: node.asConcreteType))>", terminator: "")
   }
   func visit(_ token: TokenSyntax) -> SyntaxVisitorContinueKind {
     print("<\(type(of: token))>", terminator: "")