[libc++] [P0879] constexpr std::sort

This completes libc++'s implementation of
P0879 "Constexpr for swap and swap related functions."
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0879r0.html

For the feature-macro adjustment, see
https://cplusplus.github.io/LWG/issue3256

Differential Revision: https://reviews.llvm.org/D93661

GitOrigin-RevId: 493f1407927cdf6db2e08f150ed9f9b8abe4ac45
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 5fe7005..7028ff0 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -200,7 +200,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_concepts``                            *unimplemented*
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_algorithms``                *unimplemented*
+    ``__cpp_lib_constexpr_algorithms``                ``201806L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_complex``                   *unimplemented*
     ------------------------------------------------- -----------------