Split <stddef.h> out of <cstddef>.

There are a bunch of macros (__need_size_t etc) that request just one piece of
<stddef.h>; if any one of these is defined, we just directly include the
underlying header.

Note that <stddef.h> provides a ::nullptr_t. We don't want that available to
includers of <cstddef>, so instead of following the usual pattern where <cfoo>
includes <foo.h> then pulls things from :: into std:: with using-declarations,
we implement <stddef.h> and <cstddef> separately; both include <__nullptr> for
the definition of std::nullptr_t.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@249761 91177308-0d34-0410-b5e6-96231b3b80d8

include/stddef.h

@@ -0,0 +1,56 @@
+// -*- C++ -*-
+//===--------------------------- stddef.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.
+//
+//===----------------------------------------------------------------------===//
+
+#if defined(__need_ptrdiff_t) || defined(__need_size_t) || \
+    defined(__need_wchar_t) || defined(__need_NULL) || defined(__need_wint_t)
+#include_next <stddef.h>
+
+#elif !defined(_LIBCPP_STDDEF_H)
+#define _LIBCPP_STDDEF_H
+
+/*
+    stddef.h synopsis
+
+Macros:
+
+    offsetof(type,member-designator)
+    NULL
+
+Types:
+
+    ptrdiff_t
+    size_t
+    max_align_t
+    nullptr_t
+
+*/
+
+#include <__config>
+#include_next <stddef.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+#ifdef __cplusplus
+
+extern "C++" {
+#include <__nullptr>
+using std::nullptr_t;
+}
+
+// Re-use the compiler's <stddef.h> max_align_t where possible.
+#if !defined(__CLANG_MAX_ALIGN_T_DEFINED) && !defined(_GCC_MAX_ALIGN_T)
+typedef long double max_align_t;
+#endif
+
+#endif
+
+#endif  // _LIBCPP_STDDEF_H