runtime/_mkmalloc: add a copy of cloneNode

cloneNode is defined in golang.org/x/tools/internal/astutil. Make a copy
of it so we can easily clone AST nodes.

Change-Id: I6a6a6964132e663e64faa00fa6037cf6e8d4cbc6
Reviewed-on: https://go-review.googlesource.com/c/go/+/703396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>

src/runtime/_mkmalloc/astutil/clone.go

@@ -0,0 +1,73 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a copy of golang.org/x/tools/internal/astutil/clone.go
+
+package astutil
+
+import (
+	"go/ast"
+	"reflect"
+)
+
+// CloneNode returns a deep copy of a Node.
+// It omits pointers to ast.{Scope,Object} variables.
+func CloneNode[T ast.Node](n T) T {
+	return cloneNode(n).(T)
+}
+
+func cloneNode(n ast.Node) ast.Node {
+	var clone func(x reflect.Value) reflect.Value
+	set := func(dst, src reflect.Value) {
+		src = clone(src)
+		if src.IsValid() {
+			dst.Set(src)
+		}
+	}
+	clone = func(x reflect.Value) reflect.Value {
+		switch x.Kind() {
+		case reflect.Pointer:
+			if x.IsNil() {
+				return x
+			}
+			// Skip fields of types potentially involved in cycles.
+			switch x.Interface().(type) {
+			case *ast.Object, *ast.Scope:
+				return reflect.Zero(x.Type())
+			}
+			y := reflect.New(x.Type().Elem())
+			set(y.Elem(), x.Elem())
+			return y
+
+		case reflect.Struct:
+			y := reflect.New(x.Type()).Elem()
+			for i := 0; i < x.Type().NumField(); i++ {
+				set(y.Field(i), x.Field(i))
+			}
+			return y
+
+		case reflect.Slice:
+			if x.IsNil() {
+				return x
+			}
+			y := reflect.MakeSlice(x.Type(), x.Len(), x.Cap())
+			for i := 0; i < x.Len(); i++ {
+				set(y.Index(i), x.Index(i))
+			}
+			return y
+
+		case reflect.Interface:
+			y := reflect.New(x.Type()).Elem()
+			set(y, x.Elem())
+			return y
+
+		case reflect.Array, reflect.Chan, reflect.Func, reflect.Map, reflect.UnsafePointer:
+			panic(x) // unreachable in AST
+
+		default:
+			return x // bool, string, number
+		}
+	}
+	return clone(reflect.ValueOf(n)).Interface().(ast.Node)
+}