unicode: change SimpleFold to handle invalid runes

Functions like ToLower and ToUpper return the invalid rune back,
so we might as well do the same here.

I changed my mind about panicking when I tried to document the behavior.

Fixes #16690 (again).

Change-Id: If1c68bfcd66daea160fd19948e7672b0e1add106
Reviewed-on: https://go-review.googlesource.com/30935
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/unicode/letter.go

@@ -320,6 +320,7 @@ type foldPair struct {
 // the Unicode-defined simple case folding. Among the code points
 // equivalent to rune (including rune itself), SimpleFold returns the
 // smallest rune > r if one exists, or else the smallest rune >= 0.
+// If r is not a valid Unicode code point, SimpleFold(r) returns r.
 //
 // For example:
 //	SimpleFold('A') = 'a'
@@ -331,9 +332,11 @@ type foldPair struct {
 //
 //	SimpleFold('1') = '1'
 //
+//	SimpleFold(-2) = -2
+//
 func SimpleFold(r rune) rune {
-	if r < 0 {
-		panic("unicode: negative rune is disallowed")
+	if r < 0 || r > MaxRune {
+		return r
 	}
 
 	if int(r) < len(asciiFold) {

src/unicode/letter_test.go

@@ -432,19 +432,9 @@ func TestSimpleFold(t *testing.T) {
 			r = out
 		}
 	}
-}
 
-func TestSimpleFoldPanic(t *testing.T) {
-	got := func() (r interface{}) {
-		defer func() { r = recover() }()
-		SimpleFold(-1)
-		return nil
-	}()
-	want := "unicode: negative rune is disallowed"
-
-	s, _ := got.(string)
-	if s != want {
-		t.Errorf("SimpleFold(-1) should panic, got: %q, want: %q", got, want)
+	if r := SimpleFold(-42); r != -42 {
+		t.Errorf("SimpleFold(-42) = %v, want -42", r)
 	}
 }