build: MSVC fix for unspecified std::chrono::duration type

[Radnyx]

Another patch to get the MSVC build going.

The chrono_literals operator""s actually has an unspecified return type (https://eel.is/c++draft/time.duration.literals).
So library implementations can end up with either double or long double as the duration rep. For instance,

Clang's implementation:

_LIBCPP_HIDE_FROM_ABI constexpr chrono::duration<long double> operator""s(long double __s) {
  return chrono::duration<long double>(__s);
}

MSVC's implementation:

_EXPORT_STD _NODISCARD constexpr _CHRONO duration<double> operator""s(long double _Val) noexcept
/* strengthened */ {
    return _CHRONO duration<double>(_Val);
}

I changed the assertion to grab the rep from the result of calling the operator. Looks a bit tautological, but it works.

[mpusz]

I was unaware of that. Thanks!