[Ada] Reuse First_Formal for generic subprograms

gcc/ada/

	* lib-xref.adb (Generate_Reference_To_Formals): Remove dedicated
	branch for generic subprograms (they are now handled together
	with non-generic subprograms in the ELSE branch); replace a
	low-level Ekind membership test with a high-level call to
	Is_Access_Subprogram_Type.