C#: Improve the unsafe predicate on Modifiable.

[michaelnebel]

It turns out that the unsafe keyword is discarded during compilation, which means that when extracting from referenced assemblies, we are not able to identify whether a member is unsafe.
The compiler requires methods, delegates and properties that make any use of pointer like types is declared unsafe.
As this might not be that relevant for our queries it is relevant for stub generation.

One example of a nasty delegate type declared in .NET Runtime (ie. a delegate with a delagate* (function pointer) return type).

// Generated from `System.Runtime.InteropServices.ObjectiveC.ObjectiveCMarshal+UnhandledExceptionPropagationHandler` in `System.Runtime.InteropServices, Version=7.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a`
unsafe public delegate delegate* unmanaged<System.IntPtr, void> UnhandledExceptionPropagationHandler(System.Exception exception, System.RuntimeMethodHandle lastMethod, out System.IntPtr context);

[michaelnebel]

DCA looks good.