Fix priority queue assertion which enforce the item to have strict order

[cherylEnkidu]

The crash was triggered by recent changes in standard library. In C++ priority queue library, it forces the item inside the queue to have strict order, which means when A > B, B < A is true.

Reference standard library code:

template <class _Tp, class _Up>
  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI bool operator()(_Tp& __x, _Up& __y) {
    bool __r = __comp_(__x, __y);
    if (__r)
      __do_compare_assert(0, __y, __x);
    return __r;
  }

However, in customized compare function for FieldIndex, it only compares sequence_number and collection_group, which leads to two Field Index could potentially evaluate to equal.

Hence this PR add a unique identifier to the field index to make sure they will never evaluate to equal.

The fix can be verified by running Firestore_Tests_iOS in Xcode. It should fail in main branch and success with this change.

[wu-hui]

Is it possible to create a unit test for this change?

[wu-hui]

Is it possible to create a unit test for this change?

Scratch that, yeah, there is already a test for this.