Reland [libc++] [LWG3321] Mark "year_month_day_last::day() specification does not cover !ok() values" issue as "Nothing to do", but add assertion.
This LWG issue states that the result of year_month_day_last::day() is implementation defined if ok() is false.
However, from user perspective, calling day() in this situation will lead to a (possibly difficult to find) crash.
Hence, I have added an assertion to warn user at least when assertions are enabled.
Reviewed By: ldionne, #libc
Differential Revision: https://reviews.llvm.org/D70346