cmd/compile: don't print (empty) package name in errors referring to built-ins

Fixes #14010.

Change-Id: Idfd4b063eecf453fe00f3e798099023707a65963
Reviewed-on: https://go-review.googlesource.com/18738Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>

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

@@ -419,7 +419,7 @@ func symfmt(s *Sym, flag int) string {
 	if s.Pkg != nil && flag&obj.FmtShort == 0 {
 		switch fmtmode {
 		case FErr: // This is for the user
-			if s.Pkg == localpkg {
+			if s.Pkg == builtinpkg || s.Pkg == localpkg {
 				return s.Name
 			}
 

test/fixedbugs/issue14010.go

+// errorcheck
+
+// Copyright 2016 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 built-in types don't get printed with
+// (empty) package qualification.
+
+package main
+
+func main() {
+	true = false // ERROR "cannot assign to true"
+	byte = 0     // ERROR "not an expression" "cannot assign to byte"
+}