[libc++][regex] Uses operator<=> in sub_match.

The removal of operator!= in this header will be done in a separate
commit.

Note in the synopsis of P1614R2 there is a constexpr
  template<class BiIter>
    constexpr auto operator<=>(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);

In the implementation of P1614R2 there isn't a constexpr
  template<class BiIter>
    auto operator<=>(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);

There doesn't seem to be an LWG-issue, but it was fixed in the Standard
by removing the constexpr in b050fd474f11441942c88ef69b8622c8036656ac.

Implements part of:
- P1614R2 The Mothership has Landed

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D132310