[Ada] Add Ada 2022 Key function to sets containers

[Ada] Add Ada 2022 Key function to sets containers

This patch adds the new Generic_Keys.Key function to the set children
of Ada.Containers.

gcc/ada/

	* libgnat/a-cbhase.ads, libgnat/a-cborse.ads,
	libgnat/a-cihase.ads, libgnat/a-ciorse.ads,
	libgnat/a-cohase.ads, libgnat/a-coorse.ads (Key): New function
	that takes a Container parameter, implemented as an expression
	function, so it is self explanatory (doesn't need a comment).