[CodeCompletion] Only include results from other modules once
In case of ambigous expression/global completions, we call `getValueCompletionsInDeclContext` multiple times for the amigous solutions to the constraint system. This can cause modules to be included multiple times in `RequestedCachedResults` and thus global results from these modules are reported multiple times. Make `RequestedCachedResults` a set so we don’t get duplicate results.
rdar://92048610
