[libc++] counting_semaphore should not be default-constructible.

Neither the current C++2b draft, nor any revision of [p1135],
nor libstdc++, claims that `counting_semaphore` should be
default-constructible. I think this was just a copy-paste issue
somehow.

Also, `explicit` was missing from the constructor.

Also, `constexpr` remains missing; but that's probably more of a
technical limitation, since apparently there are some platforms
where we don't (can't??) use the atomic implementation and
have to rely on pthreads, which obviously isn't constexpr.

Differential Revision: https://reviews.llvm.org/D110042