[Ada] Strengthen checks for instantiation with interface types

[Ada] Strengthen checks for instantiation with interface types

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

gcc/ada/

	* sem_ch12.adb (Instance_Exists): New function, subsidiary of
	Validate_Derived_Type_Instance, to verify that all interfaces
	implemented by the formal type are also implemented by the actual. The
	verification is complicated when an interface of the formal is declared
	in a generic unit and the actual is declared in an instance of it.
	There is currently no mechanism to relate an interface declared within
	a generic to the corresponding interface in an instance, so we must
	traverse the list of interfaces of the actual, looking for a name
	match, and verifying that that interface is declared in an instance.

From-SVN: r260726