Throw if VisualStudioWorkspace.TryApplyChanges is called from a background thread

This is something we could support one day, but until we do let's throw
a reasonable exception instead of exploding in some bizzare way.

Fixes https://github.com/dotnet/roslyn/issues/17782

src/VisualStudio/Core/Def/Implementation/ProjectSystem/VisualStudioWorkspaceImpl.cs

@@ -126,6 +126,11 @@ private bool TryGetHostProject(ProjectId projectId, out IVisualStudioHostProject
             Microsoft.CodeAnalysis.Solution newSolution,
             IProgressTracker progressTracker)
         {
+            if (_foregroundObject.IsValueCreated && !_foregroundObject.Value.IsForeground())
+            {
+                throw new InvalidOperationException(ServicesVSResources.VisualStudioWorkspace_TryApplyChanges_cannot_be_called_from_a_background_thread);
+            }
+
             var projectChanges = newSolution.GetChanges(this.CurrentSolution).GetProjectChanges().ToList();
             var projectsToLoad = new HashSet<Guid>();
             foreach (var pc in projectChanges)

src/VisualStudio/Core/Def/ServicesVSResources.Designer.cs

@@ -2314,6 +2314,15 @@ internal class ServicesVSResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to VisualStudioWorkspace.TryApplyChanges cannot be called from a background thread..
+        /// </summary>
+        internal static string VisualStudioWorkspace_TryApplyChanges_cannot_be_called_from_a_background_thread {
+            get {
+                return ResourceManager.GetString("VisualStudioWorkspace_TryApplyChanges_cannot_be_called_from_a_background_thread", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to When inserting properties, events and methods, place them:.
         /// </summary>

src/VisualStudio/Core/Def/ServicesVSResources.resx

@@ -897,4 +897,7 @@ Additional information: {1}</value>
   <data name="Remove_naming_rule" xml:space="preserve">
     <value>Remove naming rule</value>
   </data>
+  <data name="VisualStudioWorkspace_TryApplyChanges_cannot_be_called_from_a_background_thread" xml:space="preserve">
+    <value>VisualStudioWorkspace.TryApplyChanges cannot be called from a background thread.</value>
+  </data>
 </root>
\ No newline at end of file