Ada: Fix illegal Aggregate aspect not rejected

The Ada 2022 RM is adamant that the names specified in the Aggregate aspect
must denote "exactly one" subprogram, in other words that it is illegal to
use names that denote more than one subprogram in the Aggregate aspect.

gcc/ada/
	PR ada/123289
	* sem_ch13.adb (Resolve_Aspect_Aggregate.Resolve_Operation): Give
	an error if the operation's name denotes more than one subprogram.

gcc/testsuite/
	* gnat.dg/specs/aggr9.ads: New test.