Add host option to disable AssemblyLoadContext

A host can disable the use of AssemblyLoadContext by returning
a value of "true" or "1" when GetHostOption is called with the
option name "DisableAssemblyLoadContext".

This is an escape hatch for hosts that have issues with the ALC
behavior, allowing them to opt out until the underlying issues
can be fixed.

Mono.TextTemplating/Mono.TextTemplating/CompiledTemplate.cs

@@ -85,7 +85,11 @@ TemplateProcessor CreateTemplateProcessor ()
 
 			// hosts are supposed to return null of they don't want to use a domain
 			// but check for CurrentDomain too so we can optimize if they do that
-			if (domain == null || domain == AppDomain.CurrentDomain) {
+			if (domain == null || domain == AppDomain.CurrentDomain
+#if FEATURE_ASSEMBLY_LOAD_CONTEXT
+			 	|| host.IsAssemblyLoadContextDisabled ()
+#endif
+			 ) {
 				return new TemplateProcessor ();
 			}
 

Mono.TextTemplating/Mono.TextTemplating/HostOptions.cs

@@ -0,0 +1,16 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using Microsoft.VisualStudio.TextTemplating;
+
+static class HostOptionExtensions
+{
+	const string DisableAlcOptionName = "DisableAssemblyLoadContext";
+
+	static bool IsOptionTrue (this ITextTemplatingEngineHost host, string optionName) =>
+		host.GetHostOption(optionName) is string optionVal
+			&& (optionVal == "1" || optionVal.Equals("true", StringComparison.OrdinalIgnoreCase));
+
+	public static bool IsAssemblyLoadContextDisabled (this ITextTemplatingEngineHost host) => host.IsOptionTrue (DisableAlcOptionName);
+}