Add stdbool.h wrapper for libc++

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.

Reviewers: mclow.lists, rsmith, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16346

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@261381 91177308-0d34-0410-b5e6-96231b3b80d8

include/stdbool.h

@@ -0,0 +1,39 @@
+// -*- 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