cmd/compile: reject identifiers beginning with non-ASCII digit

Fixes #11359.

Change-Id: I0fdfa410939f7e42020cbb19d74a67e1cc3cd610
Reviewed-on: https://go-review.googlesource.com/16919Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -1469,6 +1469,9 @@ talph:
 			if !unicode.IsLetter(r) && !unicode.IsDigit(r) && (importpkg == nil || r != 0xb7) {
 				Yyerror("invalid identifier character U+%04x", r)
 			}
+			if cp.Len() == 0 && unicode.IsDigit(r) {
+				Yyerror("identifier cannot begin with digit U+%04x", r)
+			}
 			cp.WriteRune(r)
 		} else if !isAlnum(c) && c != '_' {
 			break

test/fixedbugs/issue11359.go

+// errorcheck
+
+// Copyright 2015 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.
+
+// identifiers beginning with non-ASCII digits were incorrectly accepted.
+// issue 11359.
+
+package p
+var ۶ = 0 // ERROR "identifier cannot begin with digit"