cmd/gc: suppress array index error caused by a previously reported error

Fixes #7153

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/82180043

src/cmd/gc/typecheck.c

@@ -2525,8 +2525,9 @@ typecheckcomplit(Node **np)
 			typecheck(&l->left, Erv);
 			evconst(l->left);
 			i = nonnegconst(l->left);
-			if(i < 0) {
+			if(i < 0 && !l->left->diag) {
 				yyerror("array index must be non-negative integer constant");
+				l->left->diag = 1;
 				i = -(1<<30);	// stay negative for a while
 			}
 			if(i >= 0)

test/fixedbugs/bug176.go

@@ -9,6 +9,6 @@ package main
 var x int
 
 var a = []int{ x: 1}	// ERROR "constant"
-var b = [...]int{ x : 1}	// ERROR "constant"
+var b = [...]int{x: 1}
 var c = map[int]int{ x: 1}
 

test/fixedbugs/issue7153.go

+// errorcheck
+
+// Copyright 2014 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.
+
+// Issue 7153: array invalid index error duplicated on successive bad values
+
+package p
+
+var _ = []int{a: true, true} // ERROR "undefined: a" "cannot use true \(type bool\) as type int in array element"