libstdc++: Fix arithmetic bug in chrono::year_month::operator+

libstdc++: Fix arithmetic bug in chrono::year_month::operator+

This fixes the months-based addition for year_month when the
year_month's month component is 0.

libstdc++-v3/ChangeLog:

	* include/std/chrono (year_month::operator+): Properly handle a
	month value of 0 by casting the month value to int before
	subtracting 1 from it so that the difference is sign-extended in
	the subsequent addition.
	* testsuite/std/time/year_month/1.cc: Test adding months to a
	year_month whose month component is below or above the
	normalized range of [1,12].