ada: Add guard before querying the type for its interfaces

ada: Add guard before querying the type for its interfaces

Fix crash on illegal code, when routine Iface_Present_In_Ancestor is
called on the predefined String type and attempts to examine the list of
interfaces.

gcc/ada/

	* sem_type.adb (Iface_Present_In_Ancestor): Only look at the list of
	interfaces for types that allow it. The guard is a high-level equivalent
	of the entity kinds listed in the preconditon of the Interfaces query.