Fix assertion failure on VSS library

gcc/ada/
	PR ada/108858
	* sem_ch6.adb (Analyze_Subprogram_Body_Helper): For functions with
	separate spec, if their return type was visible through a limited-
	with context clause, their extra formals were not added when the
	spec was analyzed.  Now the full view must be available, and the
	extra formals can be created and Returns_By_Ref computed.