ada: Assorted minor cleanups and tweaks

ada: Assorted minor cleanups and tweaks

There should be no functional changes.

gcc/ada/ChangeLog:

	* einfo.ads (Has_Homonym): Fix inaccuracy in description.
	* sem_ch8.ads (Find_Direct_Name): Remove obsolete description.
	* sem_ch12.adb (Analyze_Associations): Rename I_Node parameter
	into N and adjust description.
	(Analyze_Subprogram_Instantiation): Add missing description.
	(Contains_Instance_Of): Fix description.
	(Associations): Rename Generic_Actual_Rec into Actual_Rec and
	Gen_Assocs_Rec into Match_Rec.
	(Analyze_One_Association): Rename I_Node parameter into N.
	(Check_Fixed_Point_Warning): Rename Gen_Assocs parameter into
	Match.
	(Body of Associations): Minor cleanups and tweaks.
	(Analyze_Associations): Rename I_Node parameter into N and
	adjust implementation.
	(Analyze_One_Association): Likewise.
	(Analyze_Package_Instantiation): Remove obsolete code and clean up.
	(Check_Fixed_Point_Warning): Rename Gen_Assocs parameter into
	Match and adjust implementation.
	(Freeze_Package_Instance): Simplify condition.
	(Get_Unit_Instantiation_Node): Add support for instantiations of
	subprograms and stop the loop properly in case of errors.
	* sem_util.ads (Add_Global_Declaration): Rename N parameter into
	Decl and fix description.
	* sem_util.adb (Add_Global_Declaration): Rename N parameter into
	Decl and adjust implementation.