This patch improves the handling of an improper declaaration of aspect
First for a GNAT-defined iterable type,

2018-12-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_util.adb (Get_Actual_Subtype): Function can return type
	mark.
	(Get_Cursor_Type): Improve recovery and error message on a
	misplaced First aspect for an iterable type.

gcc/testsuite/

	* gnat.dg/iter4.adb: New testcase.

From-SVN: r267013