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.
`source.request.conformingmethods` is a new SourceKit request which
receives a source position and a list of protocol namses, returns a list
of methods whose return type conforms to the requested protocols.
rdar://problem/44699573
