Improve super calls #25362

Currently we call objc_msgSendSuper by doing something like this:

NSObject obj = ...;
objc_msgSendSuper (obj.SuperHandle, Selector.GetHandle ("..."), ...);

this is a problem, because NSObject.SuperHandle is a pointer, and the implementation has to go through some rather significant hoops to make sure:

• The pointer always points to valid memory (which can't be managed memory, because the GC can move managed memory)
• Is freed at some point
• Isn't freed too early

An improvement would be to do it more like native code does it:

NSObject obj = ...;
var super = new objc_super (obj);
unsafe {
    objc_msgSendSuper (&super, Selector.GetHandle ("..."), ...);
}
GC.KeepAlive (obj);

// and where the P/Invoke is:

unsafe static extern void objc_msgSendSuper (objc_super* super, IntPtr selector, ...);

// and objc_super's ctor is:

public objc_super (NSObject obj)
{
    Handle = obj.Handle;
    SuperHandle = obj.ClassHandle;
}

This would allow the NSObject.SuperHandle code to be simplified (and even removed in XAMCORE_5_0) - or at least less optimized. In particular it would be useful to remove the classHandle field from the NSObjectData struct, so that NSObjectData's size is no more than 2 pointers, so that we can put it in the tagged memory returned by ObjectiveCMarshal.CreateReferenceTrackingHandle (a very simple solution would be somehow chuck the return value from SuperHandle into a static ConditionalWeakTable).

The plan for objc_msgSendSuper would go something like:

• Make the NSObject.objc_super struct a top-level public type, and rename it to ObjCSuper. Also if we could make it a 'readonly ref' struct that would probably be good.
• Update bgen to generate the new pattern.