<algorithm>: Relax const-ness requirements on ranges::_Meow_bound_unchecked

[frederick-vs-ja]

Fixes #4863.

---

I guess it would be better to require const& for the checking in std::sortable to make ranges::inplace_merge able to use ranges::upper_bound like this:

template<dereferenceable I>
  struct const_lvalue_adaptor { // exposition only
    const iter_reference_t<I>& operator*(); // not defined
  };

template<class I, class R = ranges::less, class P = identity>
  concept sortable =
    permutable<I> &&
    indirect_strict_weak_order<R, const_lvalue_adaptor<projected<I, P>>>;

But at this moment I decide just not to require const& for internal checking.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for fixing these compiler errors! 🛠️ 🪄 😻