[libcxx] Implement P0883R2 ("Fixing Atomic Initialization")

Reviewed By: Quuxplusone

Differential Revision: https://reviews.llvm.org/D103769

NOKEYCHECK=True
GitOrigin-RevId: 56aac567acfd696f54163e33d8df02dc2ad3a72e
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index ff5197f..c2690e7 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -182,7 +182,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_atomic_shared_ptr``                   *unimplemented*
     ------------------------------------------------- -----------------
-    ``__cpp_lib_atomic_value_initialization``         *unimplemented*
+    ``__cpp_lib_atomic_value_initialization``         ``201911L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_atomic_wait``                         ``201907L``
     ------------------------------------------------- -----------------