cmd/gc: fix spurious 'const initializer is not a constant' error

Fixes #6403

LGTM=rsc
R=iant, rsc
CC=golang-codereviews
https://golang.org/cl/72840044

src/cmd/gc/subr.c

@@ -2243,6 +2243,7 @@ adddot(Node *n)
 	int c, d;
 
 	typecheck(&n->left, Etype|Erv);
+	n->diag |= n->left->diag;
 	t = n->left->type;
 	if(t == T)
 		goto ret;

src/cmd/gc/typecheck.c

@@ -3174,7 +3174,10 @@ typecheckdef(Node *n)
 			goto ret;
 		}
 		if(e->type != T && e->op != OLITERAL || !isgoconst(e)) {
-			yyerror("const initializer %N is not a constant", e);
+			if(!e->diag) {
+				yyerror("const initializer %N is not a constant", e);
+				e->diag = 1;
+			}
 			goto ret;
 		}
 		t = n->type;

test/fixedbugs/issue6403.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 6403: fix spurious 'const initializer is not a constant' error
+
+package p
+
+import "syscall"
+
+const A int = syscall.X // ERROR "undefined: syscall.X"
+const B int = voidpkg.X // ERROR "undefined: voidpkg"