[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``
     ------------------------------------------------- -----------------
diff --git a/docs/Status/Cxx2bPapers.csv b/docs/Status/Cxx2bPapers.csv
index c0ef688..12fb513 100644
--- a/docs/Status/Cxx2bPapers.csv
+++ b/docs/Status/Cxx2bPapers.csv
@@ -1,6 +1,6 @@
 "Paper #","Group","Paper Name","Meeting","Status","First released version"
 "`P0881R7 <https://wg21.link/P0881R7>`__","LWG","A Proposal to add stacktrace library","Autumn 2020","",""
-"`P0943R6 <https://wg21.link/P0943R6>`__","LWG","Support C atomics in C++","Autumn 2020","",""
+"`P0943R6 <https://wg21.link/P0943R6>`__","LWG","Support C atomics in C++","Autumn 2020","|Complete|","15.0"
 "`P1048R1 <https://wg21.link/P1048R1>`__","LWG","A proposal for a type trait to detect scoped enumerations","Autumn 2020","|Complete|","12.0"
 "`P1679R3 <https://wg21.link/P1679R3>`__","LWG","string contains function","Autumn 2020","|Complete|","12.0"
 "","","","","",""