ada: Add destructors extension

ada: Add destructors extension

This patch adds a GNAT-specific extension which enables "destructors".
Destructors are an optional replacement for Ada.Finalization where some
aspects of the interaction with type derivation are different.

gcc/ada/ChangeLog:

	* doc/gnat_rm/gnat_language_extensions.rst: Document new extension.
	* snames.ads-tmpl: Add name for new aspect.
	* gen_il-fields.ads (Has_Destructor, Is_Destructor): Add new fields.
	* gen_il-gen-gen_entities.adb (E_Procedure, Type_Kind): Add new fields.
	* einfo.ads (Has_Destructor, Is_Destructor): Document new fields.
	* aspects.ads: Add new aspect.
	* sem_ch13.adb (Analyze_Aspect_Specifications,
	Check_Aspect_At_Freeze_Point, Check_Aspect_At_End_Of_Declarations):
	Add semantic analysis for new aspect.
	(Resolve_Finalization_Procedure): New function.
	(Resolve_Finalizable_Argument): Use new function above.
	* sem_util.adb (Propagate_Controlled_Flags): Extend for new field.
	* freeze.adb (Freeze_Entity): Add legality check for new aspect.
	* exp_ch3.adb (Expand_Freeze_Record_Type, Predefined_Primitive_Bodies):
	Use new field.
	* exp_ch7.adb (Build_Finalize_Statements): Add expansion for
	destructors.
	(Make_Final_Call, Build_Record_Deep_Procs): Adapt to new Has_Destructor
	field.
	(Build_Adjust_Statements): Tweak to handle cases of empty lists.
	* gnat_rm.texi: Regenerate.