[Ada] Checks on instantiations with formal derived types with interfaces

[Ada] Checks on instantiations with formal derived types with interfaces

This patch implements the rule stated in RM 12.5.5 : the actual shall be
a descendant of very progenitor of the formal type.

2018-05-25  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch12.adb (Validate_Derived_Type_Instance): Verify that the actual
	for a formal derived type implements all the interfaces declared for
	the formal.

gcc/testsuite/

	* gnat.dg/interface6.adb: New testcase.

From-SVN: r260723