Use `<stdatomic.h>` with MSVC and C++

and use fallback only for C.

It fixes the isssue with clang-cl:

```
#include <stdatomic.h>
#include <stdbool.h>
#ifdef __cplusplus
#include <atomic>
using namespace std;
#endif

int main() {
    atomic_bool b = true;
}
```

```
$ clang-cl /TC main.cpp
# works
```
```
$ clang-cl /TP /std:c++20 main.cpp

stdatomic.h(70,6): error: conflicting types for 'atomic_thread_fence'
void atomic_thread_fence(memory_order);
     ^
atomic(166,24): note: previous definition is here
extern "C" inline void atomic_thread_fence(const memory_order _Order) noexcept {

...

fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.
```
Many errors but
`<stdatomic.h>` has many macros to built-in functions.
```
#define atomic_thread_fence(order) __c11_atomic_thread_fence(order)
```
and MSVC `<atomic>` has real functions.
and the built-in functions are redefined.

Reviewed By: #libc, aaron.ballman, Mordante

Differential Revision: https://reviews.llvm.org/D130419