Let stddef.h respect __need_{wchar_t, size_t, NULL, ptrdiff_t, wint_t}.

glibc expects that stddef.h only defines a single thing if either of these
defines is set.  For example, before this change, a C file containing

  #include <stdlib.h>
  int ptrdiff_t = 0;

would compile with gcc but not with clang. Now it compiles with clang too.

This also fixes PR12997, where older versions of the Linux headers would define
NULL incorrectly, and glibc would define __need_NULL and expect stddef.h to
redefine NULL with the correct definition.

llvm-svn: 207606