Need to define __STDC_VERSION_STDARG_H__ as 202311L for c23 standard

Variadic arguments with no leading arguments is one of such C23 features and is already available with -std=c23.

Since C23, standard headers introduce their own version macros like `__STDC_VERSION_HEADERNAME_H__`
See https://open-std.org/JTC1/SC22/WG14/www/docs/n3220.pdf 7.16.

Since variadic arguments with no leading arguments are already available, it would be better to define
`__STDC_VERSION_STDARG_H__`

The absence of the version macro will give developers wrong impression that the feature is unavailable.

Currently, we have a test clang/test/C/C23/n2359.c that checks that `__STDC_VERSION_STDARG_H__` is not defined.

That test-code contains the following text:
```
#include
#ifndef __STDC_VERSION_STDARG_H__
#error "__STDC_VERSION_STDARG_H__ not defined"
// expected-error@-1 {{"__STDC_VERSION_STDARG_H__ not defined"}}
#endif
```
Is there a reason why `__STDC_VERSION_STDARG_H__` macro should not be defined in stdarg.h? I will be happy to add it if approved.