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.
#endif
~~~
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
diff --git a/lib/Headers/stdatomic.h b/lib/Headers/stdatomic.h
index 665551e..1e47bcb 100644
--- a/lib/Headers/stdatomic.h
+++ b/lib/Headers/stdatomic.h
@@ -12,8 +12,12 @@
/* If we're hosted, fall back to the system's stdatomic.h. FreeBSD, for
* example, already has a Clang-compatible stdatomic.h header.
+ *
+ * Exclude the MSVC path as well as the MSVC header as of the 14.31.30818
+ * explicitly disallows `stdatomic.h` in the C mode via an `#error`. Fallback
+ * to the clang resource header until that is fully supported.
*/
-#if __STDC_HOSTED__ && __has_include_next(<stdatomic.h>)
+#if __STDC_HOSTED__ && __has_include_next(<stdatomic.h>) && !defined(_MSC_VER)
# include_next <stdatomic.h>
#else
