cmd/gc: make embedded, unexported fields read-only.

Fixes #7363.

LGTM=gri
R=gri, rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/66510044

src/cmd/gc/reflect.c

@@ -1127,7 +1127,8 @@ ok:
 					ot = dgopkgpath(s, ot, t1->sym->pkg);
 			} else {
 				ot = dgostringptr(s, ot, nil);
-				if(t1->type->sym != S && t1->type->sym->pkg == builtinpkg)
+				if(t1->type->sym != S &&
+				   (t1->type->sym->pkg == builtinpkg || !exportname(t1->type->sym->name)))
 					ot = dgopkgpath(s, ot, localpkg);
 				else
 					ot = dgostringptr(s, ot, nil);

test/fixedbugs/issue7363.go

+// run
+
+// 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 7363: CanSet must return false for unexported embedded struct fields.
+
+package main
+
+import "reflect"
+
+type a struct {
+}
+
+type B struct {
+	a
+}
+
+func main() {
+	b := &B{}
+	v := reflect.ValueOf(b).Elem().Field(0)
+	if v.CanSet() {
+		panic("B.a is an unexported embedded struct field")
+	}
+}