Suppress clone in iterator conversion

[lucic71]

From, tests/unit/redundant_copy_in_conversion:

  // Comparing const_iterator with iterator forces an implicit conversion of
  // `end` to const_iterator. The AST shape differs between platforms:
  //
  //   Linux: const_it == ConvertingCtor(end)
  //   macOS: const_it == ConvertingCtor(CopyCtor(end))
  //
  // The extra inner CopyCtor on macOS would emit a redundant .clone() in the
  // generated Rust. cpp2rust suppresses it so the output matches Linux.
  const_it == end ? 0 : 1;

This only happens on iterator types because the distinction between iterator and const_iterator. Other containers don't have this issue.