Add regression test for assembly origin for event marking (#102836)

Adds a regression test for the issue that was fixed in #102613.

The assembly level attribute marks the members on the referenced type, and the assembly is the MessageOrigin for the marking.

I confirmed this fails with before #102613.

src/tools/illink/test/Mono.Linker.Tests.Cases/Attributes/AssemblyAttributeAccessesMembers.cs

@@ -0,0 +1,66 @@
+// 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;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Mono.Linker.Tests.Cases.Attributes;
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+
+[assembly: KeptAttributeAttribute (typeof (AccessesMembersAttribute))]
+[assembly: AccessesMembers (typeof (AssemblyAttributeAccessesMembers.TypeWithMembers))]
+namespace Mono.Linker.Tests.Cases.Attributes
+{
+	/// <summary>
+	/// A regression test for the issue that was fixed by https://github.com/dotnet/runtime/pull/102613.
+	/// Events were assumed to always have a MemberDefinition or Descriptor file as the warning origin, but it also can occur from an assembly attribute.
+	/// </summary>
+	[Kept]
+	class AssemblyAttributeAccessesMembers
+	{
+		[Kept]
+		public static void Main ()
+		{
+			typeof (AssemblyAttributeAccessesMembers).Assembly.GetCustomAttributes (false);
+		}
+
+		[Kept]
+		public class TypeWithMembers
+		{
+			[Kept]
+			public TypeWithMembers () { }
+
+			[Kept]
+			public void Method () { }
+
+			[Kept]
+			public int Field;
+
+			[Kept]
+			[KeptBackingField]
+			public int Property { [Kept] get; [Kept] set; }
+
+			[Kept]
+			[KeptEventAddMethod]
+			[KeptEventRemoveMethod]
+			[KeptBackingField]
+			public event EventHandler Event;
+		}
+	}
+
+	[Kept]
+	[KeptBaseType (typeof (Attribute))]
+	public class AccessesMembersAttribute : Attribute
+	{
+		[Kept]
+		public AccessesMembersAttribute (
+			[KeptAttributeAttribute (typeof (DynamicallyAccessedMembersAttribute))]
+			[DynamicallyAccessedMembers (DynamicallyAccessedMemberTypes.All)]
+			Type type)
+		{
+		}
+	}
+}