Update CircularBuffer erase(iter) to return the entry after the one that is replaced

[benoy-sifive]

No description provided.

[ghost]

Ugh... I thought this would be a quick fix. CB is totally broken with respect to iterators. You can't even do this:

  auto next_it = circ_buf.begin();
  ++next_it;
  circ_buf.erase(circ_buf.begin());
  assert(next_it.isValid()); // WILL INCORRECTLY ASSERT!!!