[Ada] Dangling cursor checks in Element function

[Ada] Dangling cursor checks in Element function

In Ada.Containers.Ordered_Maps, if a dangling cursor is passed to the Element
function, execution is erroneous. Therefore, the compiler is not obligated to
detect this error. However, this patch inserts code that will detect this error
in some cases, and raise Program_Error. The same applies to Ordered_Sets,
Ordered_Multisets, Indefinite_Ordered_Maps, Indefinite_Ordered_Sets, and
Indefinite_Ordered_Multisets. No test available for erroneous execution.

2018-06-11  Bob Duff  <duff@adacore.com>

gcc/ada/

	* libgnat/a-ciorma.adb, libgnat/a-ciormu.adb, libgnat/a-ciorse.adb,
	libgnat/a-coorma.adb, libgnat/a-coormu.adb, libgnat/a-coorse.adb:
	(Element): Add code to detect dangling cursors in some cases.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@261424 138bc75d-0d04-0410-961f-82ee72b054a4