Include a bit in DocumentId to indicate if it corresponds to a SG document or not

[CyrusNajmabadi]

Currently, the IDE has to always go down an expensive path when asked for teh Document corresponding to a particular DocumentId. This is because we don't know if the doc-id refers to a real user-documetn or a potential SG document. As such, if we don't find the real document, we have to go generate all the SG docs, even if they may not match teh doc id either.

This changes the internal representation of a DocId to include if it was synthesized to correspond to an SG doc or not. If not, we can shortcircuit those calls immediately, returning that no doc was found at all after hte fast user-code lookup. If it is an sg doc id, then we do go and do the more expensive work like we did before.

[ToddGrun]

revoking review

[ToddGrun]

[JoeRobich]

@CyrusNajmabadi This PR broke OmniSharp's ability to create DocumentIds for source generated documents. I will expose the new CreateFromSerlialized method from the O# EA, but wondered if perhaps the IsSourceGenerated property could be a nullable bool. In the null case the document id would be treated with the legacy behavior.

[CyrusNajmabadi]

@JoeRobich ick :)

1. we should have an EA layer for omnisharp if they need this.
2. omnisharp should def pass along the bit saying if a doc is source generated or not (i didn't even know omnisharp could make source generated docs).
3. It's def weird for anything other than us to be making source generated docs :)
4. i don't want a nullable bool here. This is really core functionality that is valuable (we can add a lot of asserts/fast-paths with this bit). I would prefer everything just move onto it.

LMK what you thnik.

[JoeRobich]

@CyrusNajmabadi It isn't that we are making source generated docs but requesting them for use with GoToDef, etc... (see https://github.com/OmniSharp/omnisharp-roslyn/blob/22424c7a9635e5de0794ad97854ce37e7fdda1ad/src/OmniSharp.Roslyn.CSharp/Services/Navigation/SourceGeneratedFileService.cs#L37). O# has an EA lib and I was planning on updating it. Didn't know if there are potentially other users of this API who might break. grep.app doesn't know of many https://grep.app/search?q=DocumentId.CreateFromSerialized, github finds a few more https://github.com/search?q=DocumentId.CreateFromSerialized+language%3AC%23&type=code&l=C%23