ada: Repair support for user-defined literals in arithmetic operators

ada: Repair support for user-defined literals in arithmetic operators

It was partially broken to fix a regression in error reporting, because the
fix was applied to the first pass of resolution instead of the second pass,
as needs to be done for user-defined literals.

gcc/ada/

	* sem_ch4.ads (Unresolved_Operator): New procedure.
	* sem_ch4.adb (Has_Possible_Literal_Aspects): Rename into...
	(Has_Possible_User_Defined_Literal): ...this.  Tidy up.
	(Operator_Check): Accept again unresolved operators if they have a
	possible user-defined literal as operand.  Factor out the handling
	of the general error message into...
	(Unresolved_Operator): ...this new procedure.
	* sem_res.adb (Resolve): Be prepared for unresolved operators on
	entry in Ada 2022 or later.  If they are still unresolved on exit,
	call Unresolved_Operator to give the error message.
	(Try_User_Defined_Literal): Tidy up.