[XC] Consider compiling the OnIdiomExtension into OnIdiom<T>

[simonrozsival]

This is a follow-up to #21927 (see #21927 (comment)).

The {OnIdiom ...} XAML markup extension is not trimmable because it needs to call Activator.CreateInstance(type) with a type that cannot be annotated with [DynamicallyAccessedMembers].

In the cases when XamlC can infer the type of the value the extension is holding (T), the non-generic {OnIdiom ...} extension can be transformed into a generic instance of OnIdiom<T> which is trimming friendly. We can also consider producing warnings for the cases when the compiler is not able to infer T and prompt the developer to transform the extension into OnIdiom<T> manually.

Example 1

Consider this example:

<Label Text="{OnIdiom Default='Default Idiom', Phone='Phone', Tablet='Tablet', Desktop='Desktop', TV='TV', Watch='Watch'}" />

In this case, the compiler should be able to tell that all the values are strings and that the target property type is also a string. The transformed XAML should look like this:

<Label>
  <Label.Text>
    <OnIdiom x:TypeArguments="x:String">
        <OnIdiom.Default>Default Idiom</OnIdiom.Default>
        <!-- ... -->
    </OnIdiom>
  </Label.Text>
</Label>

Example 2

In the following example (src/ProfiledAot/src/maui/App.xaml), there is a mismatch between the target property type and the type of the OnIdiom default value:

<CollectionView Margin="{OnIdiom Default=1,1,1,1}" />

The compiler should be able to tell that the Margin property is of type Thickness which has a [TypeConverter(...)] attribute and so it can generate the IL to construct OnIdiom<Thickness> and use the TypeConverter to convert the string values into the target type.

/cc @StephaneDelcroix