[Ada] Incorrect accessibility checking on aliased formals

[Ada] Incorrect accessibility checking on aliased formals

2020-06-03  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* libgnat/a-cborse.adb, libgnat/a-cihase.adb,
	libgnat/a-ciorse.adb, libgnat/a-coorse.adb: Modified to use
	'Unrestricted_Access in certain cases where static accessibility
	errors were triggered.
	* exp_ch6.adb (Expand_Simple_Return_Statement): Add generation
	of dynamic accessibility checks as determined by
	Is_Special_Aliased_Formal_Access.
	* sem_attr.adb (Resolve_Attribute): Add call to
	Is_Special_Aliased_Formal_Access to avoid performing static
	checks where dynamic ones are required.
	* sem_ch6.adb (Check_Return_Obj_Accessibility): Handle renamed
	objects within component associations requiring special
	accessibility checks.
	* sem_util.adb, sem_util.ads (Is_Special_Aliased_Formal_Access):
	Created to detect the special case where an aliased formal is
	being compared against the level of an anonymous access return
	object.
	(Object_Access_Level): Remove incorrect condition leading to
	overly permissive accessibility levels being returned on
	explicitly aliased parameters.