[clang][modules] giving the __stddef_ headers their own modules can cause redeclaration errors with -fbuiltin-headers-in-system-modules (#84127)

On Apple platforms, some of the stddef.h types are also declared in
system headers. In particular NULL has a conflicting declaration in
<sys/_types/_null.h>. When that's in a different module from
<__stddef_null.h>, redeclaration errors can occur.

Make the \_\_stddef_ headers be non-modular in
-fbuiltin-headers-in-system-modules and restore them back to not
respecting their header guards. Still define the header guards though.
__stddef_max_align_t.h was in _Builtin_stddef_max_align_t prior to the
addition of _Builtin_stddef, and it needs to stay in a module because
struct's can't be type merged. __stddef_wint_t.h didn't used to have a
module, but leave it in it current module since it doesn't really belong
to stddef.h.

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