ILLink: Stop giving special treatment to icalls

As discussed dotnet#113437 (comment)

Author: mrvoorhe

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

@@ -3149,7 +3149,7 @@ protected virtual void ProcessMethod (MethodDefinition method, in DependencyInfo
 			MarkCustomAttributes (method.MethodReturnType, new DependencyInfo (DependencyKind.ReturnTypeAttribute, method), methodOrigin);
 			MarkMarshalSpec (method.MethodReturnType, new DependencyInfo (DependencyKind.ReturnTypeMarshalSpec, method), methodOrigin);
 
-			if (method.IsPInvokeImpl || method.IsInternalCall) {
+			if (method.IsPInvokeImpl) {
 				ProcessInteropMethod (method, methodOrigin);
 			}
 

src/tools/illink/test/ILLink.RoslynAnalyzer.Tests/Interop/InternalCalls/InternalCallsTests.cs

@@ -45,5 +45,11 @@ public Task UnusedFieldsOfTypesPassedByRefAreNotRemoved ()
 		{
 			return RunTest ();
 		}
+
+		[Fact]
+		public Task NoSpecialMarking ()
+		{
+			return RunTest ();
+		}
 	}
 }

src/tools/illink/test/Mono.Linker.Tests.Cases/Interop/InternalCalls/DefaultConstructorOfReturnTypeIsNotRemoved.cs

@@ -0,0 +1,36 @@
+// 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 System.Runtime.CompilerServices;
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+
+namespace Mono.Linker.Tests.Cases.Interop.InternalCalls;
+
+public class NoSpecialMarking
+{
+	public static void Main ()
+	{
+		A a = null;
+		SomeMethod (ref a, null, null);
+	}
+
+	[Kept]
+	class A;
+
+	[Kept]
+	class B;
+
+	[Kept]
+	class C;
+
+	[Kept]
+	class D
+	{
+		int field1;
+		int field2;
+	}
+
+	[Kept]
+	[MethodImpl (MethodImplOptions.InternalCall)]
+	static extern C SomeMethod (ref A a, B b, D d);
+}