Iteration routines should come in pairs, i.e. First_Formal with
Next_Formal and First_Entity with Next_Entity. This patch fixes two
occurrences where First_Formal was used with Next_Entity. Cleanup only;
semantics is unaffected.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch12.adb (Check_Abstract_Primitives): Match First_Formal
	with Next_Formal.
	* sem_ch6.adb (Is_Non_Overriding_Operation): Likewise.