ada: Reject structural instantiation in generic formal part

The structural instance would amount to a formal package in this context,
and that's totally unsupported by the current implementation.

gcc/ada/ChangeLog:

	* sem_ch3.adb (Analyze_Private_Extension_Declaration): Be prepared
	for Find_Type_Of_Subtype_Indic returning no type.
	* sem_ch4.adb (Process_Generic_Instantiation): Give an error if the
	generic instantation is referenced in a generic formal part.
	* sem_ch12.adb (Analyze_Formal_Derived_Type): Return early in every
	case of serious errors.