Fix error message when a property named color is override

It said `Cannot override property 'background'` because color is defined
as below

```slint
property <color> color <=> background;
```

For the warning, the name should not be resolved.

Fixes #6324

internal/compiler/object_tree.rs

@@ -1013,7 +1013,7 @@ impl Element {
                 Type::Invalid => {} // Ok to proceed with a new declaration
                 _ => {
                     diag.push_error(
-                        format!("Cannot override property '{}'", prop_name),
+                        format!("Cannot override property '{}'", unresolved_prop_name),
                         &prop_decl
                             .DeclaredIdentifier()
                             .child_token(SyntaxKind::Identifier)

internal/compiler/tests/syntax/basic/property_declaration.slint

@@ -25,4 +25,7 @@ export Test := Comp {
 
     property<bool> pressed;
 //                 ^error{Cannot declare property 'pressed' when a callback with the same name exists}
+
+    property<color> color;
+//                  ^error{Cannot override property 'color'}
 }