cmd/gc: silence assignment errors to undefined symbols

Fixes #6406.

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/46900043

src/cmd/gc/typecheck.c

@@ -2680,6 +2680,11 @@ checkassign(Node *n)
 		n->etype = 1;
 		return;
 	}
+
+	// have already complained about n being undefined
+	if(n->op == ONONAME)
+		return;
+
 	yyerror("cannot assign to %N", n);
 }
 

test/fixedbugs/issue6406.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.
+
+package main
+
+func main() {
+	s = "bob" // ERROR "undefined.*s"
+	_ = s // ERROR "undefined.*s"
+}

test/typecheck.go

@@ -14,5 +14,5 @@ func mine(int b) int {	// ERROR "undefined.*b"
 
 func main() {
 	mine()		// GCCGO_ERROR "not enough arguments"
-	c = mine()	// ERROR "undefined.*c|not enough arguments" "cannot assign to c"
+	c = mine()	// ERROR "undefined.*c|not enough arguments"
 }