Improve argument completion when Intellicode is not available

[genlu]

Based on feedback from #6374 (comment)

I have the same problem, argument completion is so useless, no matter what type of function parameter, it always suggests by the alphabet of class, variables, namespace, etc...

Some options we might want to consider:

1. disable argument completion if IntelliCode is not available (via C# devkit)
2. improvement default selection to match inferred argument type.