test: check that surrogate pair runes and huge rune values are rejected

R=r
CC=golang-dev
https://golang.org/cl/1615042

test/char_lit1.go

+// errchk $G -e $F.go
+
+// Copyright 2009 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.
+
+package main
+
+const (
+	// check that surrogate pair elements are invalid
+	// (d800-dbff, dc00-dfff).
+	_ = '\ud7ff' // ok
+	_ = '\ud800'  // ERROR "Unicode"
+	_ = "\U0000D999"  // ERROR "Unicode"
+	_ = '\udc01' // ERROR "Unicode"
+	_ = '\U0000dddd'  // ERROR "Unicode"
+	_ = '\udfff' // ERROR "Unicode"
+	_ = '\ue000' // ok
+	_ = '\U0010ffff'  // ok
+	_ = '\U00110000'  // ERROR "Unicode"
+	_ = "abc\U0010ffffdef"  // ok
+	_ = "abc\U00110000def"  // ERROR "Unicode"
+	_ = '\Uffffffff'  // ERROR "Unicode"
+)
+