Fix sub_match <=> basic_string with custom traits/allocators

[StephanTLavavej]

Found by std/re/re.submatch/re.submatch.op/compare.pass.cpp in the upcoming libcxx update.

This follows WG21-N4964 [re.submatch.op]/5:

template<class BiIter, class ST, class SA>
  auto operator<=>(
      const sub_match<BiIter>& lhs,
      const basic_string<typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

Returns:

static_cast<SM-CAT(BiIter)>(lhs.compare(
    typename sub_match<BiIter>::string_type(rhs.data(), rhs.size()))
      <=> 0
    )

without constructing a temporary. Our equality operators are already correct.

[StephanTLavavej]

I'm speculatively mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.