[Ada] Implement new legality rules introduced in C.6(13) by AI12-0128

[Ada] Implement new legality rules introduced in C.6(13) by AI12-0128

2019-12-16  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst (VFA): Document
	extension of the no-aliasing rule to any subcomponent.
	* freeze.adb (Freeze_Object_Declaration): Small comment tweak.
	(Freeze_Record_Type): Do not deal with delayed aspect
	specifications for components here but...
	(Freeze_Entity): ...here instead.
	* sem_ch12.adb (Instantiate_Object): Improve wording of errors
	given for legality rules in C.6(12) and implement the new rule
	in C.6(13).
	* sem_res.adb (Resolve_Actuals): Likewise.
	* sem_prag.adb (Check_Atomic_VFA): New procedure implementing
	the new legality rules in C.6(13).
	(Process_Atomic_Independent_Shared_Volatile): Call
	Check_Atomic_VFA to check the legality rules.  Factor out code
	marking types into...
	(Mark_Type): ...this new procedure.
	(Check_VFA_Conflicts): Do not check the legality rules here.
	(Pragma_Atomic_Components): Call Check_Atomic_VFA on component
	type.
	* sem_util.ads (Is_Subcomponent_Of_Atomic_Object): Declare.
	* sem_util.adb (Is_Subcomponent_Of_Atomic_Object): New
	predicate.
	* gnat_rm.texi: Regenerate.

From-SVN: r279412