[-Wunsafe-buffer-usage] Check count-attributed objects that are used and assigned in the same group

[patrykstefanski]

Check if the bounds-attributed group has assignments to objects that are also used in the same group. In those cases, the correctness of the group might depend on the order of assignments. We conservatively disallow such assignments.

In the example below, the bounds-check in sp.first() uses the value of b before the later update, which can lead to OOB if b was less than 42.

  void foo(int *__counted_by(a + b) p, int a, int b, std::span<int> sp) {
    p = sp.first(b + 42).data();
    b = 42; // b is assigned and used
    a = b;
  }

rdar://161608319