[Ada] Fix handling of inherited discriminant constraints

[Ada] Fix handling of inherited discriminant constraints

2018-07-17  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_util.adb (Gather_Components): A discriminant of an ancestor may
	have been constrained by a later ancestor, so when looking for the
	value of that hidden discriminant we must examine the stored constraint
	of other ancestors than the immediate parent type.

gcc/testsuite/

	* gnat.dg/discr54.adb, gnat.dg/discr54_pkg.ads: New testcase.

From-SVN: r262767