[libc++] Implement P1391 for string_view

Implement P1391 (https://wg21.link/p1391) which allows
`std::string_view` to be constructible from any contiguous range of
characters.

Note that a different paper (http://wg21.link/P1989) handles the generic
range constructor for `std::string_view`.

Reviewed By: ldionne, Quuxplusone, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D110718