libstdc++/ranges: make _RangeAdaptorClosure befriend operator|

libstdc++/ranges: make _RangeAdaptorClosure befriend operator|

This declares the range adaptor pipe operators a friend of the
_RangeAdaptorClosure base class so that the std module doesn't need to
export them for ADL to find them.

Note that we deliberately don't define these pipe operators as hidden
friends, see r14-3293-g4a6f3676e7dd9e.

libstdc++-v3/ChangeLog:

	* include/std/ranges (views::__adaptor::_RangeAdaptorClosure):
	Befriend both operator| overloads.
	* src/c++23/std.cc.in: Don't export views::__adaptor::operator|.

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