[Ada] Crash with private types and renamed discriminants

[Ada] Crash with private types and renamed discriminants

This patch fixes a compiler abort on an object declaration whose type
is a private type with discriminants, and whose full view is a derived
type that renames some discriminant of its parent.

2018-05-22  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch3.adb (Search_Derivation_Levels): Whenever a parent type is
	private, use the full view if available, because it may include renamed
	discriminants whose values are stored in the corresponding
	Stored_Constraint.

gcc/testsuite/

	* gnat.dg/discr49.adb, gnat.dg/discr49_rec1.adb,
	gnat.dg/discr49_rec1.ads, gnat.dg/discr49_rec2.adb,
	gnat.dg/discr49_rec2.ads: New testcase.

From-SVN: r260521