Fix bad assembly when a nested exported type is marked via link.xml

When a nested exported type is marked via link.xml and the declaring type is not marked, cecil will write out an invalid exported type table which will lead to exceptions such as
```
System.ArgumentOutOfRangeException : Specified argument was out of the range of valid values.
   at Mono.Collections.Generic.Collection`1.get_Item(Int32 index)
   at Mono.Cecil.MetadataReader.ReadExportedTypes()
   at Mono.Cecil.ModuleDefinition.<>c.<get_ExportedTypes>b__116_0(ModuleDefinition _, MetadataReader reader)
   at Mono.Cecil.ModuleDefinition.Read[TItem,TRet](TRet& variable, TItem item, Func`3 read)
   at Mono.Cecil.ModuleDefinition.get_ExportedTypes()
   at Mono.Cecil.MetadataResolver.GetType(ModuleDefinition module, TypeReference reference)
   at Mono.Cecil.MetadataResolver.Resolve(TypeReference type)
   at Mono.Linker.Tests.TestCasesRunner.TestResolver.TryResolve(TypeReference typeReference) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Trimming.Tests.Shared\TestResolver.cs:line 12
   at Mono.Linker.ModuleDefinitionExtensions.ResolveType(ModuleDefinition module, String typeFullName, ITryResolveMetadata resolver) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\src\linker\Linker\ModuleDefinitionExtensions.cs:line 50
   at Mono.Linker.TypeNameResolver.<ResolveTypeName>g__GetSimpleTypeFromModule|6_0(TypeName typeName, ModuleDefinition module) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\src\linker\Linker\TypeNameResolver.cs:line 135
   at Mono.Linker.TypeNameResolver.ResolveTypeName(AssemblyDefinition originalAssembly, TypeName typeName, List`1 typeResolutionRecords) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\src\linker\Linker\TypeNameResolver.cs:line 101
   at Mono.Linker.TypeNameResolver.TryResolveTypeName(AssemblyDefinition assembly, String typeNameString, TypeReference& typeReference, List`1& typeResolutionRecords) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\src\linker\Linker\TypeNameResolver.cs:line 44
   at Mono.Linker.Tests.TestCasesRunner.ResultChecker.VerifyLinkingOfOtherAssemblies(AssemblyDefinition original) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Mono.Linker.Tests\TestCasesRunner\ResultChecker.cs:line 320
   at Mono.Linker.Tests.TestCasesRunner.ResultChecker.Check(TrimmedTestCaseResult linkResult) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Mono.Linker.Tests\TestCasesRunner\ResultChecker.cs:line 114
   at Mono.Linker.Tests.TestCases.All.Run(TestCase testCase) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Mono.Linker.Tests\TestCases\TestSuites.cs:line 305
   at Mono.Linker.Tests.TestCases.All.TypeForwardingTests(TestCase testCase) in E:\UnitySrc\dev\unity-runtime-1\src\tools\illink\test\Mono.Linker.Tests\TestCases\TestSuites.cs:line 262
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr)
```

Or if you were to try and open the linked `Forwarder.dll` without the fix in something like ILSpy, it would crash ILSpy.

An easy way to fix this seems to be marking the declaring type.

src/tools/illink/src/linker/Linker.Steps/DescriptorMarker.cs

@@ -103,6 +103,18 @@ void MarkAndPreserveAll (TypeDefinition type, XPathNavigator nav)
 		protected override TypeDefinition? ProcessExportedType (ExportedType exported, AssemblyDefinition assembly, XPathNavigator nav)
 		{
 			_context.MarkingHelpers.MarkExportedType (exported, assembly.MainModule, new DependencyInfo (DependencyKind.XmlDescriptor, _xmlDocumentLocation), GetMessageOriginForPosition (nav));
+
+			// If a nested exported type is marked, then the declaring type must also be marked otherwise cecil will write out an invalid exported type table
+			// and anything that tries to read the assembly with cecil will crash
+			if (exported.DeclaringType != null) {
+				var currentType = exported.DeclaringType;
+				while (currentType != null) {
+					var parent = currentType.DeclaringType;
+					_context.MarkingHelpers.MarkExportedType (currentType, assembly.MainModule, new DependencyInfo(DependencyKind.DeclaringType, currentType), GetMessageOriginForPosition (nav));
+					currentType = parent;
+				}
+			}
+
 			return base.ProcessExportedType (exported, assembly, nav);
 		}
 

src/tools/illink/test/Mono.Linker.Tests.Cases/TypeForwarding/Dependencies/ForwardedNestedTypeLibrary.cs

@@ -0,0 +1,25 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+
+#if INCLUDE_FORWARDERS
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo (typeof (Mono.Linker.Tests.Cases.TypeForwarding.Dependencies.ForwardedNestedTypeLibrary))]
+// [assembly: System.Runtime.CompilerServices.TypeForwardedTo (typeof (Mono.Linker.Tests.Cases.TypeForwarding.Dependencies.ForwardedNestedTypeLibrary.Nested))]
+#endif
+
+#if INCLUDE_REFERENCE_IMPL
+namespace Mono.Linker.Tests.Cases.TypeForwarding.Dependencies;
+
+public class ForwardedNestedTypeLibrary
+{
+	public class NestedOne
+	{
+		public class NestedTwo
+		{
+			public class NestedThree
+			{
+			}
+		}
+	}
+}
+#endif

src/tools/illink/test/Mono.Linker.Tests.Cases/TypeForwarding/NestedTypeForwarder.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Metadata;
+
+namespace Mono.Linker.Tests.Cases.TypeForwarding;
+
+[SetupCompileBefore ("Forwarder.dll", new[] { "Dependencies/ForwardedNestedTypeLibrary.cs" }, defines: new[] { "INCLUDE_REFERENCE_IMPL" })]
+[SetupCompileBefore ("Implementation.dll", new[] { "Dependencies/ForwardedNestedTypeLibrary.cs" }, defines: new[] { "INCLUDE_REFERENCE_IMPL" })]
+[SetupCompileAfter ("Forwarder.dll", new[] { "Dependencies/ForwardedNestedTypeLibrary.cs" }, references: new[] { "Implementation.dll" }, defines: new[] { "INCLUDE_FORWARDERS" })]
+[SetupLinkerDescriptorFile("NestedTypeForwarder.xml")]
+[KeptAssembly("Forwarder.dll")]
+[KeptTypeInAssembly("Forwarder.dll", "Mono.Linker.Tests.Cases.TypeForwarding.Dependencies.ForwardedNestedTypeLibrary")]
+[KeptTypeInAssembly("Forwarder.dll", "Mono.Linker.Tests.Cases.TypeForwarding.Dependencies.ForwardedNestedTypeLibrary/NestedOne")]
+[KeptTypeInAssembly("Forwarder.dll", "Mono.Linker.Tests.Cases.TypeForwarding.Dependencies.ForwardedNestedTypeLibrary/NestedOne/NestedTwo")]
+[KeptTypeInAssembly("Forwarder.dll", "Mono.Linker.Tests.Cases.TypeForwarding.Dependencies.ForwardedNestedTypeLibrary/NestedOne/NestedTwo/NestedThree")]
+public class NestedTypeForwarder
+{
+	public static void Main ()
+	{
+
+	}
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/TypeForwarding/NestedTypeForwarder.xml

@@ -0,0 +1,6 @@
+<linker>
+  <assembly fullname="Forwarder, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null">
+    <type fullname="Mono.Linker.Tests.Cases.TypeForwarding.Dependencies.ForwardedNestedTypeLibrary/NestedOne/NestedTwo/NestedThree" preserve="all">
+    </type>
+  </assembly>
+</linker>