[libc++][P0943] Add stdatomic.h header.

* https://wg21.link/P0943
* https://eel.is/c++draft/stdatomic.h.syn

This is a re-application of 5d1c1a24, which was reverted in 987c7f407
because it broke the LLDB build.

Co-authored-by: Marek Kurdej <marek.kurdej@gmail.com>

Differential Revision: https://reviews.llvm.org/D97044

NOKEYCHECK=True
GitOrigin-RevId: 586efd52b93f083d095bf3319da7e42f221c3f4a
diff --git a/include/module.modulemap b/include/module.modulemap
index c291901..65e628d 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -50,6 +50,10 @@
       header "setjmp.h"
       export *
     }
+    module stdatomic_h {
+      header "stdatomic.h"
+      export *
+    }
     // FIXME: <stdalign.h> is missing.
     // <signal.h> provided by C library.
     // <stdarg.h> provided by compiler.