Add stdbool.h wrapper for libc++

[EricWF]

Summary

According to the C++ standard <stdbool.h> isn't allowed to define true false or bool. However these macros are sometimes defined by the compilers stdbool.h.

Clang defines the macros whenever __STRICT_ANSI__ isn't defined (ie -std=gnu++11).
New GCC versions define the macros in C++03 mode only, older GCC versions (4.9 and before) always define the macros.

This patch adds a wrapper header for stdbool.h that undefs the required macros.

[EricWF]

Got approval from @mclow.lists offline.

include/stdbool.h

+// -*- C++ -*-
+//===--------------------------- stdbool.h --------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_STDBOOL_H
+#define _LIBCPP_STDBOOL_H
+
+
+/*
+    stdbool.h synopsis
+
+Macros:
+
+    __bool_true_false_are_defined
+
+*/
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+#include_next <stdbool.h>
+
+#ifdef __cplusplus
+#undef bool
+#undef true
+#undef false
+#undef __bool_true_false_are_defined
+#define __bool_true_false_are_defined 1
+#endif
+
+#endif  // _LIBCPP_STDBOOL_H