[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/include/version b/include/version
index 77183e5..9b37c30 100644
--- a/include/version
+++ b/include/version
@@ -294,7 +294,7 @@
 #   define __cpp_lib_char8_t                            201811L
 # endif
 // # define __cpp_lib_concepts                             202002L
-// # define __cpp_lib_constexpr_algorithms                 201806L
+# define __cpp_lib_constexpr_algorithms                 201806L
 // # define __cpp_lib_constexpr_complex                    201711L
 # define __cpp_lib_constexpr_dynamic_alloc              201907L
 # define __cpp_lib_constexpr_functional                 201907L