SwiftSyntax: add pre and post visit function so that client can override a general visiting logic. SR-6902 (#14365)

Author: nkcsgexi

tools/SwiftSyntax/SyntaxRewriter.swift.gyb

@@ -40,7 +40,18 @@ open class SyntaxRewriter {
   open func visit(_ token: TokenSyntax) -> Syntax {
     return token
   }
+
+  /// The function called before visiting the node and its descendents.
+  ///   - node: the node we are about to visit.
+  open func visitPre(_ node: Syntax) {}
+
+  /// The function called after visting the node and its descendents.
+  ///   - node: the node we just finished visiting.
+  open func visitPost(_ node: Syntax) {}
+
   public func visit(_ node: Syntax) -> Syntax {
+    visitPre(node)
+    defer { visitPost(node) }
     switch node.raw.kind {
     case .token: return visit(node as! TokenSyntax)
 % for node in SYNTAX_NODES:
@@ -70,7 +81,17 @@ open class SyntaxVisitor {
 
   open func visit(_ token: TokenSyntax) {}
 
+  /// The function called before visiting the node and its descendents.
+  ///   - node: the node we are about to visit.
+  open func visitPre(_ node: Syntax) {}
+
+  /// The function called after visting the node and its descendents.
+  ///   - node: the node we just finished visiting.
+  open func visitPost(_ node: Syntax) {}
+
   public func visit(_ node: Syntax) {
+    visitPre(node)
+    defer { visitPost(node) }
     switch node.raw.kind {
     case .token: visit(node as! TokenSyntax)
 % for node in SYNTAX_NODES: