When a derived type DT has an untagged private parent type PT with a
discriminant, where the full type of PT is tagged, and DT inherits a
function F with an anonymous access result that designates the type, the
compiler wrongly reports an error saying that DT must be declared
abstract or F overridden. A test is added to exclude checking the
abstract overriding rules that should only apply to inherited
subprograms of tagged derived types.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch3.adb (Check_Abstract_Overriding): If the type is
	derived from an untagged type, then don't perform any of the
	abstract overriding error checks.