Trigger "Save As..." for virtual document editors when saving

[Gruntfuggly]

I'm creating a document using TextDocumentContentProvider. This works well, however I would also like to be able to save the document.

• the context menu has Save greyed out
• the File menu allows me to click Save, but does nothing
• the File menu Save As... does work

I think in all three cases it should simply do the Save As functionality.

[bpasero]

I am pushing a change that should enable the flow of "File > Save" (or CtrlCmd+S) to bring up the save dialog even for virtual documents. I hope this does not break any existing flows, so may have to revisit this during the milestone.

[bpasero]

Verification: open a readonly document from a virtual document provider (for example stepping into internal source when debugging or F1 > Startup Performance). Verify you can CtrlCmd+S to save the document and a meaningful name is presented. Once you save, verify the file is opened (similar to saving an untitled file).