libstdc++: Fix use of inaccessible private member in split_view (PR93936)

libstdc++: Fix use of inaccessible private member in split_view (PR93936)

We are calling _OuterIter::__current from _InnerIter::operator==, but the former
is private within this non-member friend.  Fix this by calling
_OuterIter::operator== instead, which does the right thing here.

libstdc++-v3/ChangeLog:

	PR libstdc++/93936
	* include/std/ranges (split_view::_InnerIter::operator==): Compare
	the operands' _M_i rather than their _M_i.current().
	* testsuite/std/ranges/adaptors/split.cc: Augment test.