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.