The invocation of the code completion second pass should be implementation detail of `CompletionInstance`. Create a method on `CompletionInstance` that correctly invokes the second pass and just reutnrs the type context info results to the caller.
We had some situations left that neither returned an error, nor called the callback with results in `performOperation`. Return an error in these and adjust the tests to correctly match the error.
This refactors a bunch of code-completion methods around `performOperation` to return their results via a callback only instead of the current mixed approach of indicating failure via a return value, returning an error string as an inout parameter and success results via a callback. The new guarantee should be that the callback is always called exactly once on control flow graph.
Other than a support for passing the (currently unused) cancelled state through the different instance, there should be no functionality change.
