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.

llvm-svn: 249761