ada: sem.adb.process_bodies_in_context: check if spec has lib body before use

ada: sem.adb.process_bodies_in_context: check if spec has lib body before use

Inspector testing shows that calling Body_Lib_Unit on Spec can sometimes
fail due to the following assertion failing:

            pragma Assert
              (Unit (N) in N_Lib_Unit_Declaration_Id
                | N_Lib_Unit_Renaming_Declaration_Id);

Indeed, Unit (N) may sometimes be an N_Subprogram_Body instead of an
N_Lib_Unit_Declaration_Id.

gcc/ada/ChangeLog:

	* sem.adb (Process_Bodies_In_Context): check that Spec's unit is
	an N_Lib_Unit_Declaration_Id.