Support ExperimentalAttribute in XAML (#20765)

external/XamlX

@@ -1 +1 @@
-Subproject commit c32d3040e536ae9768233ea5a445697632578bd0
+Subproject commit 009d4815470cf4bf71d1adbb633a5d81dcb2bb52

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlDiagnosticCodes.cs

@@ -40,9 +40,9 @@ internal static class AvaloniaXamlDiagnosticCodes
     // Reserved 5000-9998
     public const string Obsolete = "AVLN5001";
 
-    internal static string XamlXDiagnosticCodeToAvalonia(object xamlException)
+    internal static string XamlXDiagnosticCodeToAvalonia(object codeOrException)
     {
-        return xamlException switch
+        return codeOrException switch
         {
             XamlXWellKnownDiagnosticCodes wellKnownDiagnosticCodes => wellKnownDiagnosticCodes switch
             {
@@ -50,6 +50,9 @@ internal static class AvaloniaXamlDiagnosticCodes
                 _ => throw new ArgumentOutOfRangeException()
             },
 
+            // ExperimentalAttribute reports its own code
+            string code => code,
+
             XamlDataContextException => DataContextResolvingError,
             XamlBindingsTransformException => BindingsError,
             XamlPropertyPathException => PropertyPathError,