[Ada] Spurious error private subtype derivation

[Ada] Spurious error private subtype derivation

This patch fixes a spurious error on a derived type declaration whose
subtype indication is a subtype of a private type whose full view is a
constrained discriminated type.

2019-07-01  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch3.adb (Build_Derived_Record_Type): If the parent type is
	declared as a subtype of a private type with an inherited
	discriminant constraint, its generated full base appears as a
	record subtype, so we need to retrieve its oen base type so that
	the inherited constraint can be applied to it.

gcc/testsuite/

	* gnat.dg/derived_type6.adb, gnat.dg/derived_type6.ads: New
	testcase.

From-SVN: r272879