Move RazorRemoteHostClient functionality to ExternalAccess.Razor (#43770)

* Move RazorRemoteHostClient functionality to ExternalAccess.Razor

* Add file

src/Tools/ExternalAccess/Razor/RazorRemoteHostClient.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.Experiments;
+using Microsoft.CodeAnalysis.Options;
+using Microsoft.CodeAnalysis.Remote;
+
+namespace Microsoft.CodeAnalysis.ExternalAccess.Razor
+{
+    internal sealed class RazorRemoteHostClient
+    {
+        private readonly RemoteHostClient _client;
+        private readonly string _serviceName;
+
+        internal RazorRemoteHostClient(RemoteHostClient client, string serviceName)
+        {
+            _client = client;
+            _serviceName = serviceName;
+        }
+
+        public static async Task<RazorRemoteHostClient?> CreateAsync(Workspace workspace, CancellationToken cancellationToken = default)
+        {
+            var clientFactory = workspace.Services.GetRequiredService<IRemoteHostClientService>();
+            var client = await clientFactory.TryGetRemoteHostClientAsync(cancellationToken).ConfigureAwait(false);
+            return client == null ? null : new RazorRemoteHostClient(client, GetServiceName(workspace));
+        }
+
+        public Task<Optional<T>> TryRunRemoteAsync<T>(string targetName, Solution? solution, IReadOnlyList<object?> arguments, CancellationToken cancellationToken)
+            => _client.TryRunRemoteAsync<T>(_serviceName, targetName, solution, arguments, callbackTarget: null, cancellationToken);
+
+        #region support a/b testing. after a/b testing, we can remove all this code
+
+        private static string? s_lazyServiceName = null;
+
+        private static string GetServiceName(Workspace workspace)
+        {
+            if (s_lazyServiceName == null)
+            {
+                var x64 = workspace.Options.GetOption(OOP64Bit);
+                if (!x64)
+                {
+                    x64 = workspace.Services.GetRequiredService<IExperimentationService>().IsExperimentEnabled(
+                        WellKnownExperimentNames.RoslynOOP64bit);
+                }
+
+                Interlocked.CompareExchange(
+                    ref s_lazyServiceName, x64 ? "razorLanguageService64" : "razorLanguageService", null);
+            }
+
+            return s_lazyServiceName;
+        }
+
+        public static readonly Option<bool> OOP64Bit = new Option<bool>(
+            nameof(InternalFeatureOnOffOptions), nameof(OOP64Bit), defaultValue: false,
+            storageLocations: new LocalUserProfileStorageLocation(InternalFeatureOnOffOptions.LocalRegistryPath + nameof(OOP64Bit)));
+
+        private static class InternalFeatureOnOffOptions
+        {
+            internal const string LocalRegistryPath = @"Roslyn\Internal\OnOff\Features\";
+        }
+
+        #endregion
+    }
+}

src/VisualStudio/Razor/RazorLanguageServiceClient.cs

@@ -15,6 +15,7 @@
 namespace Microsoft.VisualStudio.LanguageServices.Razor
 {
     // Used in https://github.com/aspnet/AspNetCore-Tooling/tree/master/src/Razor/src/Microsoft.VisualStudio.LanguageServices.Razor/OOPTagHelperResolver.cs
+    [Obsolete("Use Microsoft.CodeAnalysis.ExternalAccess.Razor.RazorRemoteHostClient instead")]
     internal sealed class RazorLanguageServiceClient
     {
         private readonly RemoteHostClient _client;

src/VisualStudio/Razor/RazorLanguageServiceClientFactory.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis;
@@ -11,6 +12,7 @@
 
 namespace Microsoft.VisualStudio.LanguageServices.Razor
 {
+    [Obsolete("Use Microsoft.CodeAnalysis.ExternalAccess.Razor.RazorRemoteHostClient instead")]
     internal static class RazorLanguageServiceClientFactory
     {
         public static async Task<RazorLanguageServiceClient> CreateAsync(Workspace workspace, CancellationToken cancellationToken = default)