required for #172974

We would like a IFileWorkingCopy implementation that is related to UntitledFileWorkingCopy as in, there is no file on disk backing the model so it wouldn't auto-save and would be recovered from a hot exit, but can more easily be discarded without prompting to save.

• If a user closes a window without onExitAndWindowClose hot exit enabled, the user would not be prompted to save
• The dirty indicator would not show for that file type