[libc++] mark barrier constructor as explicit in <barrier>

If I read the standard correctly, the public constructor of "barrier" should be marked as "constexpr explicit".
I see some of the internal classes used by the barrier header are correctly marked but I think, if I'm not mistaken, the standard would like the public class to have the correct definition as well.

Because the implementation that llvm uses by default is not constexpr friendly at this time, this revision will focus on only marking it as explicit.

Reviewed By: #libc, philnik, Mordante

Spies: philnik, Mordante, libcxx-commits

Differential Revision: https://reviews.llvm.org/D154590