Add MetadataUpdateOriginalTypeAttribute (#69751)

Fixes https://github.com/dotnet/runtime/issues/66222

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -761,6 +761,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\MethodImplAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\MethodImplOptions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ModuleInitializerAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\MetadataUpdateOriginalTypeAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\ReferenceAssemblyAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PoolingAsyncValueTaskMethodBuilder.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\CompilerServices\PoolingAsyncValueTaskMethodBuilderT.cs" />

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/MetadataUpdateOriginalTypeAttribute.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime.CompilerServices;
+
+[AttributeUsage(System.AttributeTargets.Class | System.AttributeTargets.Struct,
+                AllowMultiple=false, Inherited=false)]
+public class MetadataUpdateOriginalTypeAttribute : Attribute
+{
+    /// <summary> This attribute is emited by Roslyn when a type that is marked with (or derives
+    /// from a type that is marked with) <see
+    /// cref="System.Runtime.CompilerServices.CreateNewOnMetadataUpdateAttribute" /> is updated
+    /// during a hot reload session.  The <see cref="OriginalType" /> points to the original version
+    /// of the updated type.  The next update of the type will have the same <see
+    /// cref="OriginalType" />. Frameworks that provide support for hot reload by implementing a
+    /// <see cref="System.Reflection.Metadata.MetadataUpdateHandlerAttribute" /> may use this
+    /// attribute to relate an updated type to its original version.  </summary>
+    ///
+    /// <param name="originalType">The original type that was updated</param>
+    public MetadataUpdateOriginalTypeAttribute(Type originalType) => OriginalType = originalType;
+    /// Gets the original version of the type that this attribtue is attached to.
+    public Type OriginalType { get; }
+}

src/libraries/System.Runtime.Loader/ref/System.Runtime.Loader.cs

@@ -30,6 +30,13 @@ namespace System.Runtime.CompilerServices
     public sealed class CreateNewOnMetadataUpdateAttribute : System.Attribute
     {
     }
+    [AttributeUsage(System.AttributeTargets.Class | System.AttributeTargets.Struct,
+                    AllowMultiple=false, Inherited=false)]
+    public class MetadataUpdateOriginalTypeAttribute : Attribute
+    {
+	public MetadataUpdateOriginalTypeAttribute(Type originalType) { throw null; }
+	public Type OriginalType { get { throw null; } }
+    }
 }
 namespace System.Runtime.Loader
 {