Headers: exclude `#include_next <stdatomic.h>` on MSVC

The 14.31.30818 toolset has the following in the `stdatomic.h`:
 #ifndef __cplusplus
 #error <stdatomic.h> is not yet supported when compiling as C, but this is planned for a future release.

This results in clang failing to build existing code which relied on
`stdatomic.h` in C mode on Windows.  Simply fallback to the clang header
until that header is available as a complete implementation.

GitOrigin-RevId: 1ad7de9e92bc2977698e5f6d6493202b50c912d5
1 file changed