Update atomic feature macros, synopsis, signatures to match C++20. Improve test coverage for non-lock-free atomics.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: fc4bff0cd37fa84ee74e6dff7170b643df3ffa42
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index f5c6e5b..6177338 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -170,8 +170,20 @@
     -------------------------------------------------------------------
     ``__cpp_lib_array_constexpr``                     ``201811L``      
     ------------------------------------------------- -----------------
+    ``__cpp_lib_atomic_flag_test``                    ``201907L``      
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_atomic_float``                        *unimplemented*  
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_atomic_lock_free_type_aliases``       ``201907L``      
+    ------------------------------------------------- -----------------
     ``__cpp_lib_atomic_ref``                          *unimplemented*  
     ------------------------------------------------- -----------------
+    ``__cpp_lib_atomic_shared_ptr``                   *unimplemented*  
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_atomic_value_initialization``         *unimplemented*  
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_atomic_wait``                         ``201907L``      
+    ------------------------------------------------- -----------------
     ``__cpp_lib_bind_front``                          *unimplemented*  
     ------------------------------------------------- -----------------
     ``__cpp_lib_bit_cast``                            *unimplemented*