libstdc++: Fix capturing of lvalue references in_RangeAdaptor::operator()

This fixes a dangling-reference issue with views::split and other multi-argument
adaptors that may take its extra arguments by reference.

When creating the _RangeAdaptorClosure in _RangeAdaptor::operator(), we
currently capture all provided arguments by value.  When we then use the
_RangeAdaptorClosure and call it with a range, as in e.g.

    v = views::split(p)(range),

we forward the range and the captures to the underlying adaptor routine.  But
then when the temporary _RangeAdaptorClosure goes out of scope, the by-value
captures get destroyed and the references to these captures in the resulting view
become dangling.

This patch fixes this problem by capturing lvalue references by reference in
_RangeAdaptorClosure::operator(), and then forwarding the captures appropriately
to the underlying adaptor routine.

libstdc++-v3/ChangeLog:

	* include/std/ranges (views::__adaptor::__maybe_refwrap): New utility
	function.
	(views::__adaptor::_RangeAdaptor::operator()): Add comments.  Use
	__maybe_refwrap to capture lvalue references by reference, and then use
	unwrap_reference_t to forward the by-reference captures as references.
	* testsuite/std/ranges/adaptors/split.cc: Augment test.
	* testsuite/std/ranges/adaptors/split_neg.cc: New test.

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