Make array<const T, 0> non-CopyAssignable and make swap and fill ill-formed.

The standard isn't exactly clear how std::array should handle zero-sized arrays
with const element types. In particular W.R.T. copy assignment, swap, and fill.

This patch takes the position that those operations should be ill-formed,
and makes changes to libc++ to make it so.

This follows up on commit r324182.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324185 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/containers/sequences/array/array.fill/fill.fail.cpp

@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <array>
+
+// void fill(const T& u);
+
+#include <array>
+#include <cassert>
+
+// std::array is explicitly allowed to be initialized with A a = { init-list };.
+// Disable the missing braces warning for this reason.
+#include "disable_missing_braces_warning.h"
+
+int main() {
+  {
+    typedef double T;
+    typedef std::array<const T, 0> C;
+    C c = {};
+    // expected-error@array:* {{static_assert failed "cannot fill zero-sized array of type 'const T'"}}
+    c.fill(5.5); // expected-note {{requested here}}
+  }
+}