libstdc++: Apply proposed resolution for LWG 3449 [PR95322]

libstdc++: Apply proposed resolution for LWG 3449 [PR95322]

Now that the frontend bug PR96805 is fixed, we can cleanly apply the
proposed resolution for this issue.

This slightly deviates from the proposed resolution by declaring _CI a
member of take_view instead of take_view::_Sentinel, since it doesn't
depend on anything within _Sentinel anymore.

libstdc++-v3/ChangeLog:

	PR libstdc++/95322
	* include/std/ranges (take_view::_CI): Define this alias
	template as per LWG 3449 and remove ...
	(take_view::_Sentinel::_CI): ... this type alias.
	(take_view::_Sentinel::operator==): Adjust use of _CI
	accordingly.  Define a second overload that accepts an iterator
	of the opposite constness as per LWG 3449.
	(take_while_view::_Sentinel::operator==): Likewise.
	* testsuite/std/ranges/adaptors/95322.cc: Add tests for LWG 3449.