[Ada] Complete the more extended AST traversal used in GNATprove

[Ada] Complete the more extended AST traversal used in GNATprove

Following the work on the Ada 202X feature of iterated aggregates
(AI12-061), Loop_Actions should have been listed as a semantic field of
nodes of kind N_Iterated_Component_Association.  Fix this as well as the
new extended AST traversal which should traverse these fields.

There is no impact on compilation.

2019-08-13  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_util.adb (Traverse_More_Func): Take into account
	Loop_Actions inside N_Iterated_Component_Association nodes.
	* sinfo.ads: Document correctly Loop_Actions as a field of nodes
	of kind N_Iterated_Component_Association.

From-SVN: r274332