Add ImplementationFiles to FSharpCheckFileResults

[vasily-kirichenko]

No description provided.

[vasily-kirichenko]

I ported the changed @dsyme asked for.

[dsyme]

This got integrated via git pull http://github.com/Microsoft/visualfsharp master

[alfonsogarciacaro]

@ncave This is the PR that enabled using ParseAndCheckFileInProject in Fable. Do you think any of this can be reused so we can implement the method in your FCS Fork and enable it in Fable REPL?

[ncave]

@alfonsogarciacaro Not by itself, no. What it actually requires is using the incremental builder, or some variation of it, which we didn't need to do so far in the Fable REPL, as we didn't have a use case for it. We can continue the conversation over there, so we don't preempt this thread.