[NativeAOT] Fix ILC warning for NSObject.RegisterToggleRef (#18889)

This PR implements a workaround for the following build warning:

```
ILC: Method '[Microsoft.iOS]Foundation.NSObject.RegisterToggleRef(NSObject,native int,bool)' will always throw because:
Invalid IL or CLR metadata in 'Void Foundation.NSObject.RegisterToggleRef(Foundation.NSObject, IntPtr, Boolean)'
```

Ref #18524

tools/dotnet-linker/AppBundleRewriter.cs

@@ -1220,6 +1220,20 @@ namespace Xamarin.Linker {
 			}
 		}
 
+#if NET
+		public bool TryGet_NSObject_RegisterToggleRef (out MethodDefinition? md)
+		{
+			// the NSObject.RegisterToggleRef method isn't present on all platforms (for example on Mac)
+			try {
+				_ = GetMethodReference (PlatformAssembly, Foundation_NSObject, "RegisterToggleRef", "Foundation.NSObject::RegisterToggleRef", predicate: null, out md);
+				return true;
+			} catch (InvalidOperationException) {
+				md = null;
+				return false;
+			}
+		}
+#endif
+
 		public void SetCurrentAssembly (AssemblyDefinition value)
 		{
 			current_assembly = value;

tools/dotnet-linker/Steps/ManagedRegistrarStep.cs

@@ -168,6 +168,11 @@ namespace Xamarin.Linker {
 				Annotations.Mark (md);
 			}
 
+			// TODO: Move this to a separate "MakeEverythingWorkWithNativeAOTStep" linker step
+			if (App.XamarinRuntime == XamarinRuntime.NativeAOT && Configuration.Profile.IsProductAssembly (assembly)) {
+				ImplementNSObjectRegisterToggleRefMethodStub ();
+			}
+
 			abr.ClearCurrentAssembly ();
 		}
 
@@ -1376,5 +1381,23 @@ namespace Xamarin.Linker {
 
 			return clonedCtor;
 		}
+
+		void ImplementNSObjectRegisterToggleRefMethodStub ()
+		{
+			// The NSObject.RegisterToggleRef method is a Mono icall that is unused in NativeAOT.
+			// The method isn't included on all platforms but when it is present, we need to modify it
+			// so that ILC can trim it and it doesn't report the following warning:
+			// 
+			//    ILC: Method '[Microsoft.iOS]Foundation.NSObject.RegisterToggleRef(NSObject,native int,bool)' will always throw because:
+			//         Invalid IL or CLR metadata in 'Void Foundation.NSObject.RegisterToggleRef(Foundation.NSObject, IntPtr, Boolean)'
+			//
+			if (abr.TryGet_NSObject_RegisterToggleRef (out var registerToggleRef)) {
+				registerToggleRef!.IsPublic = false;
+				registerToggleRef!.IsInternalCall = false;
+
+				registerToggleRef!.CreateBody (out var il);
+				il.Emit (OpCodes.Ret);
+			}
+		}
 	}
 }