Re-add some Legacy ITagHelperResolver infrastructure.

- This is required until the TagHelperProjectSystem is completed.

src/Microsoft.VisualStudio.LanguageServices.Razor/Legacy/ITagHelperResolver.cs

@@ -0,0 +1,18 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Razor;
+
+namespace Microsoft.VisualStudio.LanguageServices.Razor
+{
+    // ----------------------------------------------------------------------------------------------------
+    // NOTE: This is only here for VisualStudio binary compatibility. This type should not be used; instead
+    // use the Microsoft.CodeAnalysis.Razor variant from Microsoft.CodeAnalysis.Razor.Workspaces
+    // ----------------------------------------------------------------------------------------------------
+    public interface ITagHelperResolver
+    {
+        Task<TagHelperResolutionResult> GetTagHelpersAsync(Project project);
+    }
+}

src/Microsoft.VisualStudio.LanguageServices.Razor/Legacy/LegacyTagHelperResolver.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.ComponentModel.Composition;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Razor;
+
+namespace Microsoft.VisualStudio.LanguageServices.Razor
+{
+    // ----------------------------------------------------------------------------------------------------
+    // NOTE: This is only here for VisualStudio binary compatibility. This type should not be used; instead
+    // use TagHelperResolver.
+    // ----------------------------------------------------------------------------------------------------
+    [Export(typeof(ITagHelperResolver))]
+    internal class LegacyTagHelperResolver : DefaultTagHelperResolver, ITagHelperResolver
+    {
+        [ImportingConstructor]
+        public LegacyTagHelperResolver(
+            [Import(typeof(VisualStudioWorkspace))] Workspace workspace)
+            : base(workspace.Services.GetRequiredService<ErrorReporter>(), workspace)
+        {
+        }
+    }
+}