[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