[Ada] tech debt: Parent (Empty) is not allowed

[Ada] tech debt: Parent (Empty) is not allowed

gcc/ada/

	* atree.adb, atree.ads (Parent, Set_Parent): Assert node is
	Present.
	(Copy_Parent, Parent_Kind): New helper routines.
	* gen_il-gen.adb: Add with clause.
	* nlists.adb (Parent): Assert Parent of list is Present.
	* aspects.adb, checks.adb, exp_aggr.adb, exp_ch6.adb,
	exp_util.adb, lib-xref-spark_specific.adb, osint.ads,
	sem_ch12.adb, sem_ch13.adb, sem_ch3.adb, sem_ch6.adb,
	sem_dim.adb, sem_prag.adb, sem_res.adb, sem_util.adb,
	treepr.adb: Do not call Parent and Set_Parent on the Empty node.
	* libgnat/a-stwiun__shared.adb, libgnat/a-stzunb__shared.adb:
	Minor: Fix typos in comments.
	* einfo.ads: Minor comment update.
	* sinfo-utils.ads, sinfo-utils.adb (Parent_Kind, Copy_Parent):
	New functions.