[Trimming] Fix ILLink warnings in source generated code

[simonrozsival]

Description of Change

ILLink Trim analyzer is reporting the following warnings:

/.../MyMauiApp/obj/Release/net9.0-maccatalyst/maccatalyst-arm64/Microsoft.Maui.Controls.SourceGen/Microsoft.Maui.Controls.SourceGen.CodeBehindGenerator/Resources_Styles_Colors.xaml.sg.cs(30,4): warning IL2026: Using member 'Microsoft.Maui.Controls.Xaml.Extensions.LoadFromXaml<TXaml>(TXaml, Type)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Loading XAML at runtime might require types and members that cannot be statically analyzed. Make sure all of the required types and members are preserved. [/.../MyMauiApp/MyMauiApp.csproj::TargetFramework=net9.0-maccatalyst]

It is safe to suppress the warnings in the InitializeComponent method because its method body will be replaced with XamlC in Release builds.

Issues Fixed

Contributes to #19397

/cc @jonathanpeppers @vitek-karas @StephaneDelcroix

[simonrozsival]

This is probably not the right fix. XamlC runs before ILLink so by the time ILLink processes the assembly, the InitializeComponent body should already be replaced with the compiled IL. I'm closing the PR until I figure out what exactly is going on.

[simonrozsival]

I assumed that the warnings are produced by ILLink, but they are in fact produced by Csc (or by an analyzer during C# compilation). This PR is needed after all.

[jonathanpeppers]

I think source suppression would probably work:

• dotnet new maui-lib
• Have one view or page with XAML
• dotnet build -c Release -> would run XamlC
• Any consuming app would have no warnings?

[simonrozsival]

Yes, XamlC will replace the body of InitializeComponent in that case.