Merge branch 'modelThreading' of https://github.com/CyrusNajmabadi/roslyn into modelThreading

Author: CyrusNajmabadi

src/EditorFeatures/Core.Wpf/SignatureHelp/Controller.Session_ComputeModel.cs

@@ -53,6 +53,7 @@ private async Task<Model> ComputeModelInBackgroundAsync(
                 {
                     using (Logger.LogBlock(FunctionId.SignatureHelp_ModelComputation_ComputeModelInBackground, cancellationToken))
                     {
+                        this.Computation.ThreadingContext.ThrowIfNotOnBackgroundThread();
                         cancellationToken.ThrowIfCancellationRequested();
 
                         var document = Controller.DocumentProvider.GetDocument(caretPosition.Snapshot, cancellationToken);

src/EditorFeatures/Core.Wpf/SignatureHelp/Controller.Session_SetModelSelectedItem.cs

@@ -28,6 +28,7 @@ private void SetModelExplicitlySelectedItem(Func<Model, SignatureHelpItem> selec
                 Model? model,
                 Func<Model, SignatureHelpItem> selector)
             {
+                this.Computation.ThreadingContext.ThrowIfNotOnBackgroundThread();
                 if (model == null)
                     return null;