[Ada] Hashed container Cursor type predefined equality non-conformance

[Ada] Hashed container Cursor type predefined equality non-conformance

gcc/ada/

	* libgnat/a-cohase.ads (Cursor): Synchronize comments for the Cursor
	type definition to be consistent with identical definitions in other
	container packages. Add additional comments regarding the importance of
	maintaining the "Position" component for predefined equality.
	* libgnat/a-cohama.ads (Cursor): Likewise.
	* libgnat/a-cihama.ads (Cursor): Likewise.
	* libgnat/a-cohase.adb (Find, Insert): Ensure that Cursor objects
	always have their "Position" component set to ensure predefined
	equality works as required.
	* libgnat/a-cohama.adb (Find, Insert): Likewise.
	* libgnat/a-cihama.adb (Find, Insert): Likewise.

gcc/testsuite/

	* gnat.dg/containers2.adb: New test.