OrderedDictionary `updateValue` autoclosure for insertion index

[andrews05]

I'm using the following OrderedDictionary function:

  public mutating func updateValue<R>(
    forKey key: Key,
    insertingDefault defaultValue: @autoclosure () -> Value,
    at index: Int,
    with body: (inout Value) throws -> R
  )

The defaultValue is an autoclosure, which is great, but it would be helpful if the index was also an autoclosure to prevent a possibly expensive index calculation if the key already exists.