Add `boost::unreachable`

C++11 users need an analogue of `std::unreachable`. Might be easily implemented in the way cppreference suggests:
```
[[noreturn]] inline void unreachable()
{
#if defined(_MSC_VER) && !defined(__clang__) // MSVC
    __assume(false);
#else // GCC, Clang
    __builtin_unreachable();
#endif
}
```

https://en.cppreference.com/w/cpp/utility/unreachable