go/token: add File.String method

It returns a brief, unspecified, human-readable description
of the File.

+ test, doc, relnote

Fixes #76285

Change-Id: I8b0705cf20eaac095bc9dfba7f1181774544f02c
Reviewed-on: https://go-review.googlesource.com/c/go/+/743280
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

api/next/76285.txt

@@ -0,0 +1 @@
+pkg go/token, method (*File) String() string #76285

doc/next/6-stdlib/99-minor/go/token/76285.md

@@ -0,0 +1 @@
+[File] now has a String method.

src/go/token/position.go

@@ -112,6 +112,11 @@ type File struct {
 	infos []lineInfo
 }
 
+// String returns a brief description of the File.
+func (f *File) String() string {
+	return fmt.Sprintf("%s(%d-%d)", f.Name(), f.Base(), f.End())
+}
+
 // Name returns the file name of file f as registered with AddFile.
 func (f *File) Name() string {
 	return f.name

src/go/token/position_test.go

@@ -651,3 +651,11 @@ func TestFile_End(t *testing.T) {
 		t.Errorf("Base, End = %s, want %s", got, want)
 	}
 }
+
+func TestFile_String(t *testing.T) {
+	f := NewFileSet().AddFile("a.go", 100, 42)
+	got := f.String()
+	if want := "a.go(100-142)"; got != want {
+		t.Errorf("String = %q, want %q", got, want)
+	}
+}