libstdc++: Implement missing operator overloads in max_size_type.h
This implements operator++, operator-- and operator<=> for the
integer-class types defined in max_size_type.h, which I overlooked
when originally implementing the class.
libstdc++-v3/ChangeLog:
* include/bits/max_size_type.h (__max_size_type::operator _Tp):
Fix formatting.
(__max_size_type::operator++): Define.
(__max_size_type::operator--): Likewise.
(__max_size_type::operator<=>): Conditionally define (in place
of the other comparison operators).
(__max_diff_type::operator _Tp): Fix formatting.
(__max_diff_type::operator++): Define.
(__max_diff_type::operator--): Likewise.
(__max_diff_type::operator<=>): Conditionally define (in place
of the other comparison operators).
* testsuite/std/ranges/iota/max_size_type.cc (test01): Test
these operator overloads.