These had been waiting on the ability to use std::copy from constexpr code (which in turn had been waiting on the ability to use is_constant_evaluated() to switch between memmove and non-memmove implementations of std::copy). That work landed a while ago, so these algorithms can all be constexpr in C++20 now.
While I'm in the area, update the tests. (This part has scope-crept a ton.)
According to my research, here are the remaining algorithms that P0202+P0879 marked "constexpr," which are still not implemented as "constexpr" by libc++. We'd have to implement these before we could advertise the __cpp_lib_constexpr_algorithms feature-test macro:
reverse partition sort nth_element next_permutation prev_permutation push_heap pop_heap make_heap sort_heap partial_sort partial_sort_copy
Can we make this test constexpr too?