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/cstddef

@@ -34,10 +34,10 @@ Types:
 */
 
 #include <__config>
+// Don't include our own <stddef.h>; we don't want to declare ::nullptr_t.
+#include_next <stddef.h>
 #include <__nullptr>
 
-#include <stddef.h>
-
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif