[Ada] Ada2022: AI12-0143 Index attribute for entry families

[Ada] Ada2022: AI12-0143 Index attribute for entry families

gcc/ada/

	* snames.ads-tmpl (Name_Index): New attribute name.
	(Attribute_Id): Adding Attribute_Index as regular attribute.
	* sem_attr.adb (Attribute_22): Adding Attribute_Index as Ada
	2022 attribute.
	(Analyze_Index_Attribute): Check that 'Index appears in a
	pre-/postcondition aspect or pragma associated with an entry
	family.
	(Analyze_Attribute): Adding semantic analysis for 'Index.
	(Eval_Attribute): Register 'Index as can never be folded.
	(Resolve_Attribute): Resolve attribute 'Index.
	* sem_ch9.adb (Check_Wrong_Attribute_In_Postconditions): New
	subprogram.
	(Analyze_Requeue): Check that the requeue target shall not have
	an applicable specific or class-wide postcondition which
	includes an Index attribute reference.
	* exp_attr.adb (Expand_N_Attribute_Reference): Transform
	attribute Index into a renaming of the second formal of the
	wrapper built for an entry family that has contract cases.
	* einfo.ads (Is_Entry_Wrapper): Complete documentation.