libstdc++: Convert _RangeAdaptorClosure into a CRTP base [PR108827]

Using the CRTP idiom for this base class avoids bloating the size of a
pipeline when adding distinct empty range adaptor closure objects to it,
as detailed in section 4.1 of P2387R3.

But it means we can no longer define its operator| overloads as hidden
friends, since it'd mean each instantiation of _RangeAdaptorClosure
introduces its own distinct set of hidden friends.  So e.g. for the
outer | in

  x | (views::reverse | views::join)

ADL would find 6 distinct hidden operator| friends:

  two from _RangeAdaptorClosure<_Reverse>
  two from _RangeAdaptorClosure<_Join>
  two from _RangeAdaptorClosure<_Pipe<_Reverse, _Join>>

but we really only want to consider the last two.

We avoid this issue by instead defining the operator| overloads at
namespace scope alongside _RangeAdaptorClosure.  This should be fine
because the only types defined in this namespace are _RangeAdaptorClosure,
_RangeAdaptor, _Pipe and _Partial, so we don't have to worry about
unintentional ADL.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

	PR libstdc++/108827

libstdc++-v3/ChangeLog:

	* include/std/ranges (__adaptor::_RangeAdaptorClosure):
	Convert into a CRTP class template.  Move hidden operator|
	friends into namespace scope and adjust their constraints.
	(__closure::__is_range_adaptor_closure_fn): Define.
	(__closure::__is_range_adaptor_closure): Define.
	(__adaptor::_Partial): Adjust use of _RangeAdaptorClosure.
	(__adaptor::_Pipe): Likewise.
	(views::_All): Likewise.
	(views::_Join): Likewise.
	(views::_Common): Likewise.
	(views::_Reverse): Likewise.
	(views::_Elements): Likewise.
	(views::_Adjacent): Likewise.
	(views::_AsRvalue): Likewise.
	(views::_Enumerate): Likewise.
	(views::_AsConst): Likewise.
	* testsuite/std/ranges/adaptors/all.cc: Reinstate assertion
	expecting that adding empty range adaptor closure objects to a
	pipeline doesn't increase the size of a pipeline.

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