[IDE] Set solution-specific variable types as interface types

Setting the interface type of a variable, just to reset it to a null type is actually really gross. But quite a few methods further down in the generation of code completion results (such as USR generation) need to get a variable’s type and passing them along in a separate map would be really invasive. So this seems like the least bad solution to me.

lib/IDE/ExprCompletion.cpp

@@ -105,6 +105,8 @@ void ExprTypeCheckCompletionCallback::deliverResults(
   Lookup.shouldCheckForDuplicates(Results.size() > 1);
 
   for (auto &Result : Results) {
+    WithSolutionSpecificVarTypesRAII VarTypes(Result.SolutionSpecificVarTypes);
+
     Lookup.setExpectedTypes(ExpectedTypes,
                             Result.IsImplicitSingleExpressionReturn);
     Lookup.setCanCurrDeclContextHandleAsync(Result.IsInAsyncContext);