Need to support the ability to override "generators"

To speed up code generation when switching tabs in the XAML designer, the XAML "overrides" the MSBuild::Compile generator and replaces it with the XAML single file generator. We need to make official support for this, especially as build is out-of-proc and XAML won't be able to register a host object to get unsaved changes.

See CLangFileNode::GetXamlOverrideGenerator for more information.