libstdc++: Fix view adaptors for mixed-const sentinels and iterators (PR 95322)

The bug report is that transform_view's sentinel<false> cannot be
compared to its iterator<true>.  The comparison is supposed to use
operator==(iterator<Const>, sentinel<Const>) after converting
sentinel<false> to sentinel<true>. However, the operator== is a hidden
friend so is not a candidate when comparing iterator<true> with
sentinel<false>. The required conversion would only happen if we'd found
the operator, but we can't find the operator until after the conversion
happens.

A new LWG issue has been reported, but not yet assigned a number.  The
solution suggested by Casey Carter is to make the hidden friends of the
sentinel types work with iterators of any const-ness, so that no
conversions are required.

Patrick Palka observed that join_view has a similar problem and a
similar fix is used for its sentinel.

	PR libstdc++/95322
	* include/std/ranges (transform_view::_Sentinel): Allow hidden
	friends to work with _Iterator<true> and _Iterator<false>.
	(join_view::_Sentinel): Likewise.
	* testsuite/std/ranges/adaptors/95322.cc: New test.

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