[libc++] [C++2b] [P0943] Add stdatomic.h header.

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

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

NOKEYCHECK=True
GitOrigin-RevId: 5d1c1a243c4d60249444189526ac63ced7ae999c
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 8877ec0..a6c658c 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -342,7 +342,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_stacktrace``                          *unimplemented*
     ------------------------------------------------- -----------------
-    ``__cpp_lib_stdatomic_h``                         *unimplemented*
+    ``__cpp_lib_stdatomic_h``                         ``202011L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_string_contains``                     ``202011L``
     ------------------------------------------------- -----------------