Analyzer: Type used in PInvoke should report RUC on default constructor

```C#
void Test()
{
PInvokeWithResult();
}

// IL2026 - Foo
[DllImport("nonexistent")]
static extern ResultType PInvokeWithResult();

class ResultType {
[RequiresUnreferencedCode("Foo")]
public ResultType() { }
}
```

Types used in PInvoke are potentially instantiated by the runtime via default .ctors. Thus if the .ctor is annotated with RUC this should cause a warning.

This is probably a very low priority thing to fix in the analyzer, since this would be very uncommon.