[libc++] P1645 constexpr for <numeric>

Implements P1645: constexpr for <numeric> algorithms

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D90569

GitOrigin-RevId: 67c88e47bdba2578150a9034a717d6ddcace4e13
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 637a484..aa60a03 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -196,6 +196,8 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_misc``                      *unimplemented*
     ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_numeric``                   ``201911L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_swap_algorithms``           *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_utility``                   ``201811L``