[DllImportGenerator] "MarshalUsing" ignores "ref" return type

[SupinePandora43]

[GeneratedDllImport("MyDLL")]
[return: MarshalUsing(typeof(MyStringMarshaler))]
public static partial string MyFunction();

and

[GeneratedDllImport("MyDLL")]
[return: MarshalUsing(typeof(MyStringMarshaler))]
public static partial ref string MyFunction();

will return same pinvoke, ex.

[System.Runtime.InteropServices.DllImportAttribute("MyDLL", EntryPoint = "MyFunction")]
extern private static unsafe global:Type_Of_MyStringMarshaler_Value MyFunction__PInvoke__();

[jkotas]

cc @AaronRobinsonMSFT @jkoritzinsky