[Ada] Reuse Has_Defaulted_Discriminants where possible

gcc/ada/

	* exp_attr.adb, exp_ch9.adb, sem_ch3.adb: Reuse
	Has_Defaulted_Discriminants.
	* sem_ch4.adb (Analyze_Allocator): Reuse
	Has_Defaulted_Discriminants (after reordering conjuncts); remove
	redundant IF statement, whose condition is implied by
	Has_Defaulted_Discriminants.
	* sem_util.adb (Has_Defaulted_Discriminants): Has_Discriminants
	implies that the First_Discriminant is present.
	(Is_Fully_Initialized_Type): Reuse Has_Defaulted_Discriminants.