libstdc++: Fix arithmetic bug in year_month_weekday conversion [PR96713]

libstdc++: Fix arithmetic bug in year_month_weekday conversion [PR96713]

The conversion function year_month_weekday::operator sys_days computes
the offset in days from the first weekday of the month with:

 days{(index()-1)*7}
      ^~~~~~~~~~~~~  type 'unsigned'

We want the above to yield -7d when index() is 0u, but our 'days' alias
is based on long instead of int, so the conversion from unsigned to the
underlying type of 'days' instead yields a large positive value.

This patch fixes this by casting the result of index() to int so that
the initializer is sign-extended in the conversion to long.

The added testcase also verifies we do the right thing when index() == 5.

libstdc++-v3/ChangeLog:

	PR libstdc++/96713
	* include/std/chrono (year_month_weekday::operator sys_days):
	Cast the result of index() to int so that the initializer for
	days{} is sign-extended when it's converted to the underlying
	type.
	* testsuite/std/time/year_month_weekday/3.cc: New test.