Ada: Fix bogus error on inherited operation for extension of type instance

Ada: Fix bogus error on inherited operation for extension of type instance

It comes from a small discrepancy between class-wide subtypes and types:
they both have unknown discriminants, but only the latter may have
discriminants, which causes Subtypes_Statically_Match to return False.

gcc/ada/
	PR ada/83188
	* sem_eval.adb (Subtypes_Statically_Match): Deal with class-wide
	subtypes whose class-wide types have discriminants.

gcc/testsuite/
	* gnat.dg/class_wide6.ads, gnat.dg/class_wide6.adb: New test.
	* gnat.dg/class_wide6_pkg.ads: New helper.