[libc++] [NFC] Mark P0858R0 as implemented

P0858R0 seems to be implemented. D93830 sets `__cpp_lib_string_view` and D80452 sets `__cpp_lib_array_constexpr`.

Reviewed By: Quuxplusone, Mordante, ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D114344