We need to use Extended_Index for the Position parameter of the Element
function in formal vectors so it is compatible with other primitives of
the Iterable aspect.

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

gcc/ada/

	* libgnat/a-cfinve.ads (Element): Change the type of the
	Position parameter to Extended_Index.
	* libgnat/a-cfinve.adb (Element): Idem.
	* libgnat/a-cofove.ads (Element): Idem.
	* libgnat/a-cofove.adb (Element): Idem.