ada: First controlling parameter aspect

This patch adds support for a new GNAT aspect/pragma that modifies
the semantics of dispatching primitives. When a tagged type has
this aspect/pragma, only subprograms that have the first parameter
of this type will be considered dispatching primitives; this new
pragma/aspect is inherited by all descendant types.

gcc/ada/

	* aspects.ads (Aspect_First_Controlling_Parameter): New aspect.
	Defined as implementation defined aspect that has a static boolean
	value and it is converted to pragma when the value is True.
	* einfo.ads (Has_First_Controlling_Parameter): New attribute.
	* exp_ch9.adb (Build_Corresponding_Record): Propagate the aspect
	to the corresponding record type.
	(Expand_N_Protected_Type_Declaration): Analyze the inherited
	aspect to add the pragma.
	(Expand_N_Task_Type_Declaration): ditto.
	* freeze.adb (Warn_If_Implicitly_Inherited_Aspects): New
	subprogram.
	(Has_First_Ctrl_Param_Aspect): New subprogram.
	(Freeze_Record_Type): Call Warn_If_Implicitly_Inherited_Aspects.
	(Freeze_Subprogram): Check illegal subprograms of tagged types and
	interface types that have this new aspect.
	* gen_il-fields.ads (Has_First_Controlling_Parameter): New entity
	field.
	* gen_il-gen-gen_entities.adb (Has_First_Controlling_Parameter):
	The new field is a semantic flag.
	* gen_il-internals.adb (Image): Add
	Has_First_Controlling_Parameter.
	* par-prag.adb (Prag): No action for
	Pragma_First_Controlling_Parameter since processing is handled
	entirely in Sem_Prag.
	* sem_ch12.adb (Validate_Private_Type_Instance): When the generic
	formal has this new aspect, check that the actual type also has
	this aspect.
	* sem_ch13.adb (Analyze_One_Aspect): Check that the aspect is
	applied to a tagged type or a concurrent type.
	* sem_ch3.adb (Analyze_Full_Type_Declaration): Derived tagged
	types inherit this new aspect, and also from their implemented
	interface types.
	(Process_Full_View): Propagate the aspect to the full view.
	* sem_ch6.adb (Is_A_Primitive): New subprogram; used to factor
	code and also clarify detection of primitives.
	* sem_ch9.adb (Check_Interfaces): Propagate this new aspect to the
	type implementing interface types.
	* sem_disp.adb (Check_Controlling_Formals): Handle tagged type
	that has the aspect and has subprograms overriding primitives of
	tagged types that lack this aspect.
	(Check_Dispatching_Operation): Warn on dispatching primitives
	disallowed by this new aspect.
	(Has_Predefined_Dispatching_Operation_Name): New subprogram.
	(Find_Dispatching_Type): Handle dispatching functions of tagged
	types that have the new aspect.
	(Find_Primitive_Covering_Interface): For primitives of tagged
	types that have the aspect and override a primitive of a parent
	type that does not have the aspect, we must temporarily unset
	attribute First_Controlling_ Parameter to properly check
	conformance.
	* sem_prag.ads (Aspect_Specifying_Pragma): Add new pragma.
	* sem_prag.adb (Pragma_First_Controlling_Parameter): Handle new
	pragma.
	* snames.ads-tmpl (Name_First_Controlling_Parameter): New name.
	* warnsw.ads (Warn_On_Non_Dispatching_Primitives): New warning.
	* warnsw.adb (Warn_On_Non_Dispatching_Primitives): New warning;
	not set by default when GNAT_Mode warnings are enabled, nor when
	all warnings are enabled (-gnatwa).

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