Ada: Fix bogus visibility error for iterated element association with key

Ada: Fix bogus visibility error for iterated element association with key

The problem is that the Resolve_Iterated_Association procedure, unlike its
sibling Resolve_Iterated_Component_Association, preanalyzes a copy of the
specification so, in a generic context, global references cannot later be
captured.  This changes it to preanalyze the specification directly, which
requires a small adjustment during expansion.

gcc/ada/
	PR ada/124201
	* exp_aggr.adb (Expand_Iterated_Component): Replace the iteration
	variable in the key expression and iterator filter, if any.
	* sem_aggr.adb (Resolve_Iterated_Component_Association): Preanalyze
	the specification and key expression directly.

gcc/testsuite/
	* gnat.dg/generic_inst17.adb: New test.