[SwiftBindings] In TypeMetadata handle ExistentialContainers

[stephen-hawley]

In TypeMetadata.cs in TryGetTypeMetadataUncached

if (IsExistentialContainer(type)) {
    return TryGetExistentialContainerMetadata(type, out result);
}

// ...

static bool IsExistentialContainer(Type type)
{
}

static bool TryGetExistentialContainerMetadata(Type type, [NotNullWhen(true)] out TypeMetadata? result)
{
}

Note: depending on how we end up implementation ExistentialContainer, this might not be necessary. On the one hand, if every ExistentialContainer implements ISwiftObject then this won't be necessary. On the other hand if the type metadata is unique based on the contents of the existential container, then implementing ISwiftObject won't necessarily help or will create cache issues (ie, existential containers may not be allowed in the cache).