Recently, Roslyn undeprecated the CompletionChange.TextChanges API. This API gives changes in a significantly simpler manner for us to deal with, as it splits up things like imports and the actual main change element, so we can remove a whole bunch of code dedicated to finding the changed elements and mapping original source locations to the new document. The new pattern is much simpler: except for import completion, we always resolve the change up front. For most providers, this is an extremely quick call, as most providers just return the label, and for the ones that don't we can't do much about it anyway. We can then loop through the individual changes, putting changes that are not touching the current cursor location into AdditionalTextChanges. This shrinks the completion payload pretty significantly for many scenarios, and gets rid of a bunch of special handling around it. The only remaining special handling is adjusting the filter texts and snippitizing completions that want to move the cursor. I've also aligned the behavior of the Preselect flag with Roslyn's completion handler, and removed additional filtering of completion items so that they can be filtered by the client instead.

Fixes OmniSharp#2123 as well.

… in the override completion case, while having a minimal cost for the non-override case.