ILTrim: Keep BeginInvoke/EndInvoke as a pair on delegate types

Match illink MarkStep behavior: when BeginInvoke is kept on a delegate
type, also keep EndInvoke, and vice versa. This ensures the async
delegate pattern remains functional after trimming.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: MichalStrehovsky

src/coreclr/tools/ILTrim/ILTrim/DependencyAnalysis/TokenBased/MethodDefinitionNode.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using System.Reflection;
 using System.Reflection.Metadata;
@@ -101,6 +102,23 @@ static void AddInteropAllocatedType(NodeFactory factory, DependencyList dependen
                 dependencies.Add(factory.ConstructedType((EcmaType)method.OwningType), "Type with a kept constructor");
             }
 
+            var ecmaOwningType = (EcmaType)_module.GetObject(declaringType);
+            if (ecmaOwningType.IsDelegate)
+            {
+                ReadOnlySpan<byte> methodPairName = default;
+                if (reader.StringComparer.Equals(methodDef.Name, "BeginInvoke"))
+                    methodPairName = "EndInvoke"u8;
+                else if (reader.StringComparer.Equals(methodDef.Name, "EndInvoke"))
+                    methodPairName = "BeginInvoke"u8;
+
+                if (methodPairName.Length > 0)
+                {
+                    var pairMethod = ecmaOwningType.GetMethod(methodPairName, null) as EcmaMethod;
+                    if (pairMethod != null)
+                        dependencies.Add(factory.MethodDefinition(_module, pairMethod.Handle), "Delegate BeginInvoke/EndInvoke pair");
+                }
+            }
+
             // TODO-SIZE: Property/event metadata is not strictly necessary for accessor method calls —
             // it's only needed for reflection and debugger scenarios. We could skip keeping property/event
             // rows when we know they won't be accessed via reflection, producing smaller output.