internal/tokeninternal: tag AddExistingFiles for go1.24

...and use FileSet.AddExistingFiles in go1.25.

This change is necessary so that CL 675736, which changes the
representation of FileSet, can be merged.

Updates golang/go#73205

Change-Id: Ic2815130c17b7cadd3d7b55076ad8482c508c3c7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/675955
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: adonovan

internal/tokeninternal/tokeninternal_go124.go

@@ -2,10 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// TODO(rfindley): once the new AddExistingFiles API has had some time to soak
-// in std, use it in x/tools and change the condition below to !go1.25.
-
-//go:build !addexistingfiles
+//go:build !go1.25
 
 package tokeninternal
 
@@ -22,9 +19,6 @@ import (
 // AddExistingFiles adds the specified files to the FileSet if they
 // are not already present. It panics if any pair of files in the
 // resulting FileSet would overlap.
-//
-// TODO(adonovan): replace with FileSet.AddExistingFiles in go1.25,
-// which is much more efficient.
 func AddExistingFiles(fset *token.FileSet, files []*token.File) {
 
 	// This function cannot be implemented as:

internal/tokeninternal/tokeninternal_go125.go

@@ -2,10 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// TODO(rfindley): once the new AddExistingFiles API has had some time to soak
-// in std, use it here behind the go1.25 build tag.
-
-//go:build addexistingfiles
+//go:build go1.25
 
 package tokeninternal
 
@@ -14,8 +11,6 @@ import "go/token"
 // AddExistingFiles adds the specified files to the FileSet if they
 // are not already present. It panics if any pair of files in the
 // resulting FileSet would overlap.
-//
-// TODO(adonovan): eliminate when go1.25 is always available.
 func AddExistingFiles(fset *token.FileSet, files []*token.File) {
 	fset.AddExistingFiles(files...)
 }