cmd/gc: fix spurious 'use of untyped nil' error

Fixes #6402

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/81340044

src/cmd/gc/const.c

@@ -1144,7 +1144,10 @@ defaultlit(Node **np, Type *t)
 		}
 		if(n->val.ctype == CTNIL) {
 			lineno = lno;
+			if(!n->diag) {
 				yyerror("use of untyped nil");
+				n->diag = 1;
+			}
 			n->type = T;
 			break;
 		}

test/fixedbugs/issue6402.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 6402: spurious 'use of untyped nil' error
+
+package p
+
+func f() uintptr {
+	return nil // ERROR "cannot use nil as type uintptr in return argument"
+}