Revert "[libc++] [LWG3321] Mark "year_month_day_last::day() specification does...

Revert "[libc++] [LWG3321] Mark "year_month_day_last::day() specification does not cover !ok() values" issue as "Nothing to do", but add assertion."

This reverts commit e25a2601.
It was failing due to _LIBCPP_ASSERT throwing in a method marked noexcept.