ada: Errors on legal container aggregates with iterated_element_associations

ada: Errors on legal container aggregates with iterated_element_associations

The compiler rejects various cases of container aggregates with
iterated_element_associations that include a loop_parameter_subtype_indication
or that include the "reverse" keyword. The fixes are in the parser, for
naccepting the syntax for these cases, as well as for properly accounting
for reverse iterators in the analyzer and expander.

gcc/ada/

	* exp_aggr.adb
	(Expand_Container_Aggregate.Expand_Iterated_Component): Set the
	Reverse_Present flag when creating the loop's iteration_scheme.
	* gen_il-gen-gen_nodes.adb: Add flag Reverse_Present to
	N_Iterated_Component_Association nodes.
	* par-ch3.adb (P_Constraint_Op): Remove testing for and ignoring
	of Tok_In following a constraint. It's allowed for "in" to follow
	a constraint of loop_parameter_subtype_indication of an
	iterator_specification, so it shouldn't be ignored.
	* par-ch4.adb (P_Iterated_Component_Association): Account for
	"reverse" following the "in" in an iterated_component_association,
	and set the Reverse_Present flag on the
	N_Iterated_Component_Association node. Add handling for a ":"
	following the identifier in an iterator_specification of an
	iterated_element_association, sharing the code with the "of" case
	(which backs up to the identifier at the beginning of the
	iterator_specification). Fix incorrect trailing comment following
	the call to Scan.
	(Build_Iterated_Element_Association): Set the Reverse_Present flag
	on an N_Loop_Parameter_Specification node of an
	N_Iterated_Element_Association.
	* par-ch5.adb (P_Iterator_Specification): Remove error-recovery
	and error code that reports "subtype indication is only legal on
	an element iterator", as that error can no longer be emitted (and
	was formerly only reported on one fixedbugs test).
	* sem_aggr.adb
	(Resolve_Container_Aggregate.Resolve_Iterated_Association): When
	creating an N_Iterator_Specification for an
	N_Iterated_Component_Association, set the Reverse_Present flag of
	the N_Iterated_Specification from the flag on the latter.
	* sinfo.ads: Add comments for the Reverse_Present flag, which is
	now allowed on nodes of kind N_Iterated_Component_Association.