Add typeforwarders for copied framework types

Ix.NET/Source/System.Linq.Async/System/Collections/Generic/IAsyncEnumerable.cs

@@ -26,4 +26,9 @@ namespace System.Collections.Generic
     }
 }
 
+#else
+using System.Runtime.CompilerServices;
+
+[assembly: TypeForwardedTo(typeof(System.Collections.Generic.IAsyncEnumerable<>))]
+
 #endif

Ix.NET/Source/System.Linq.Async/System/Collections/Generic/IAsyncEnumerator.cs

@@ -33,4 +33,8 @@ namespace System.Collections.Generic
     }
 }
 
+#else
+using System.Runtime.CompilerServices;
+
+[assembly: TypeForwardedTo(typeof(System.Collections.Generic.IAsyncEnumerator<>))]
 #endif

Ix.NET/Source/System.Linq.Async/System/IAsyncDisposable.cs

@@ -17,4 +17,8 @@ namespace System
     }
 }
 
+#else
+using System.Runtime.CompilerServices;
+
+[assembly: TypeForwardedTo(typeof(System.IAsyncDisposable))]
 #endif

Ix.NET/Source/System.Linq.Async/System/Runtime/CompilerServices/ConfiguredCancelableAsyncEnumerable.cs

@@ -80,4 +80,9 @@ namespace System.Runtime.CompilerServices
     }
 }
 
+#else
+using System.Runtime.CompilerServices;
+
+[assembly: TypeForwardedTo(typeof(ConfiguredCancelableAsyncEnumerable<>))]
+
 #endif