[Ada] Implementation of AI12-205: defaults for formal types

[Ada] Implementation of AI12-205: defaults for formal types

gcc/ada/

	* gen_il-fields.ads: Add Default_Subtype_Mark to enumeration
	type for fields.
	* gen_il-gen-gen_nodes.adb: Add call to create new field for
	Formal_Type_Declaration node.
	* par-ch12.adb (P_Formal_Type_Declaration): in Ada_2022 mode,
	recognize new syntax for default: "or use subtype_mark".
	(P_Formal_Type_Definition): Ditto for the case of a formal
	incomplete type.
	* sinfo.ads: Add field Default_Subtype_Mark to
	N_Formal_Type_Declaration.
	* sem_ch12.adb (Validate_Formal_Type_Default): New procedure, to
	apply legality rules to default subtypes in formal type
	declarations. Some legality rules apply to all defaults, such as
	the requirement that the default for a formal type that depends
	on previous formal entities must itself be a previously declared
	formal of the same unit. Other checks are kind- specific.
	(Analyze_Associations): Use specified default if there is no
	actual provided for a formal type in an instance.
	(Analyze_Formal_Type_Declaration): Call
	Validate_Formal_Type_Default when default subtype is present.