<xutility>: Fix inaccurate constraint of _Equal_rev_pred

[hewillk]

When no match is found, we return {false, _It1 {}}; which implies that _It1 must be a forward_iterator that can be default-constructed.

[frederick-vs-ja]

_Equal_rev_pred is only used twice in MSVC STL:

• In _Search_fn, the iterators are required to be forward_iterator:
  

  STL/stl/inc/xutility

  Line 6549 in f392449

  auto [_Match, _Mid1] = _RANGES _Equal_rev_pred(_First1, _First2, _Last2, _Pred, _Proj1, _Proj2);

• In _Find_end_fn, only random_access_iterators are used with _Equal_rev_pred:
  

  STL/stl/inc/algorithm

  Line 3064 in f392449

  _RANGES _Equal_rev_pred(_Candidate, _First2, _First2 + _Count2, _Pred, _Proj1, _Proj2);

Although the change is not strictly needed, it would be a sanity improvement to me.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for cleaning up this constraint so it makes more sense! 🧹 ✨ 😺