Implement debugger intrinsic __findnonnull

[werat]

__findnonnull is an important intrinsic used by NatVis in the CustomListItems element -- https://docs.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects?view=vs-2019#customlistitems-expansion

I couldn't find any documentation for it, but empirically it looks like this:

template <typename T>
int __findnonnull(T** ptr, int64_t size) {
  if (size < 0 || size > 100000000) {
    error("Passing in a buffer size that is negative or in excess of 100 million to __findnonnull() is not allowed.");
  }
  for (int i = 0; i < size; ++ i) {
    if (ptr[i] != nullptr) {
      return i;
    }
  }
  return -1;
}