[Generator] The generator writes bad code when one of the parameters is a T[] where T is a struct

For a code like the following, where MTLRegion is a struct:

```csharp
void ConvertSparseTileRegions (IntPtr tileRegions, IntPtr pixelRegions, MTLSize tileSize, nuint numRegions);
```

The generator writes calls as:
```csharp
var nsa_pixelRegions = NSArray.FromNSObjects (pixelRegions);
var nsa_tileRegions = NSArray.FromNSObjects (tileRegions);
```

That won't even compile because you cannot go from a struct to an NSObject.