For iterator specification appearing inside an iterated component
association, we just did ad-hoc, incomplete checks and delayed a proper
analysis until the iterated component association is expanded into loop (and
then reanalyzed).

However, when the iterated component association is not expanded, e.g.
because we are in semantic checking mode, GNATprove mode or inside a
generic, then the AST lacked any processing or error reporting.

This is fixed by reusing the existing analysis of iterator specifications,
as they also appear in other constructs, e.g. in quantified expressions.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_aggr.adb (Resolve_Iterated_Component_Association): Split
	processing of cases with and without iterator specification; reuse
	analysis of iterator specification; improve diagnostics for
	premature usage of iterator index in discrete choices.