cmd/compile: fix printing of array types in error messages

Fixes #23094.

Change-Id: I9aa36046488baa5f55cf2099e10cfb39ecd17b06
Reviewed-on: https://go-review.googlesource.com/116256
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -1268,7 +1268,7 @@ func (n *Node) exprfmt(s fmt.State, prec int, mode fmtMode) {
 
 	case OTARRAY:
 		if n.Left != nil {
-			mode.Fprintf(s, "[]%v", n.Left)
+			mode.Fprintf(s, "[%v]%v", n.Left, n.Right)
 			return
 		}
 		mode.Fprintf(s, "[]%v", n.Right) // happens before typecheck

test/fixedbugs/issue23094.go

+// errorcheck
+
+// Copyright 2018 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.
+
+// Verify that the array is reported in correct notation.
+
+package p
+
+var a [len(a)]int // ERROR "\[len\(a\)\]int"