[Ada] Crash on iterated_component_association in expression function

[Ada] Crash on iterated_component_association in expression function

This patch improves on the handling of the Ada2020 construct Iterated_
Component_Association in various contexts, when the expression involved
is a record or array aggregate.

Executing:
   gnatmake -gnatX -q main
   ./main

must yield:

   123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ

----
with Text_IO; use Text_IO;
with Exfor; use Exfor;
procedure Main is
   Map : String := Table_ASCII;
begin
   Put_Line (Map (50..91));
end;
----
package Exfor is
   function Table_ASCII return String is
      (for I in 1 .. Character'Pos (Character'Last) + 1 => Character'Val(I-1));
end Exfor;

2018-01-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_aggr.adb (Resolve_Iterated_Component_Association): Perform
	analysis on a copy of the expression with a copy of the index variable,
	because full expansion will rewrite construct into a loop with the
	original loop variable.
	* exp_aggr.adb (Gen_Assign): Defer analysis and resolution if the
	expression is an iterated component association. Full analysis takes
	place when construct is rewritten as a loop.
	(In_Place_Assign_OK, Safe_Component): An iterated_component_association
	is not safe for in-place assignment.
	* sem_util.adb (Remove_Entity): Handle properly the case of an isolated
	entity with no homonym and no other entity in the scope.

From-SVN: r256485