[libc++] std::unreachable should probably trap in some hardening modes

[ldionne]

I am not certain which modes yet, but since std::unreachable invokes UB and we want to catch UB, it would make sense to guarantee a trap there when we are in some hardening modes (e.g. certainly in the debug mode).

[shafik]

It would be nice if we could fix the more general problem: #48943

Also see: https://discourse.llvm.org/t/can-we-keep-must-progress-optimizations-around-infinite-loop-in-c-while-avoiding-some-surprising-behavior/69205/1

I realize that is much wider scope but just wanted to put it out there.