swift234

[CodeCompletion] Expose some result creation APIs for SourceKit

We want to be able to synthesize new results inside SourceKit. At this
point, the simplest way to do that is to expose the constructors for
CodeCompletionResult and a create() function for CodeCompletionString.

The expectation that any strings are stored properly inside a
CodeCompletionResultSink is documented.

Swift SVN r27822