Ada: Fix prefixed-view notation rejected for discriminated private type

The problem comes from an oversight in Analyze_Selected_Component.

gcc/ada/
	PR ada/123902
	* sem_ch4.adb (Analyze_Selected_Component): Also test
	Core_Extensions_Allowed for discriminated private types.
	Rework and augment commentary throughout the procedure.

gcc/testsuite/
	* gnat.dg/prefix4.adb: New test.
	* gnat.dg/prefix4_pkg.ads: New helper.

Co-authored-by: Liam Powell <liam@liampwll.com>