Avoid URI serialization issues in override completion by passing full text document identifier

[dibarbet]

An issue was caught by the vscode-csharp integration tests where override completion was broken with the latest roslyn on Windows. I'm not sure exactly when this regressed, but possibly related to changes made for xaml support.

I wasn't able to track down the exact cause - but on Windows the URI sent from the server in the completion resolve command is getting serialized as just the local path (e.g. not including the file:/// part). The vscode client is expecting a URI string, and when it tries to deserialize, it fails.

This inconsistency in serialization is likely because there's no explicit serializer for the Uri type defined, so it ends up being up to the whims of the serialization library. Instead, we should just pass the TextDocumentIdentifier which already defines an explicit and consistent URI serializer.

Requires client side change here - dotnet/vscode-csharp#6920