enum managed / native size

[spouliot]

The managed enum size should, when possible, match the native one. E.g.

using System;
using System.Runtime.InteropServices;
using SwiftRuntimeLibrary;
using SwiftRuntimeLibrary.SwiftMarshal;
namespace XamVersions
{
    [SwiftEnumHasRawValue(typeof(long))]
    [SwiftEnumType("libXamVersions.dylib", "_T011XamVersions8SemanticOMn", 
        "_T011XamVersions8SemanticON", "")]
    public enum Semantic
    {
        Major, 
        Minor, 
        Patch, 
        Same, 
            
            Unknown
        }
}

should be public enum Semantic : long as this will help if that same type is used in C or ObjC intro with other API. If the size don't match then extra, manual code is required (and that often leads to mistakes).

The [Native] attribute (from XI/XM) can also be used when a enum is based on n[u]int (NS[U]Integer) as we have some tooling to spot those cases.

[spouliot]

The test (minimally) and the runner (ideally, sooner than later) needs to add a reference to Xamarin.[iOS|Mac|TVOS|WatchOS].dll assemblies.

This time it's for [Native] attribute (on Int based swift enums) but it will be required for memory management and NSObject compatibility.

[spouliot]

[SwiftEnumBackingType] serve the same purpose than [Native]. The main difference is that the type is always encoded (not inferred).

In theory we don't need both, in practice there's code that use [Native] that might need to be shared.

There's also the case where type X exists for ObjC (and inside the platform assembly) where we do not want to duplicate the definition.

E.g. a NSFooEnum could be used in C API (p/invoke), ObjC API and Swift API.

We already share the same definition for C/ObjC. To share with Swift we'll need to see when other attributes, e.g. SwiftEnumType, are really required.