swift234

IRGen: Allow dynamic class layout for non-ObjC-derived types.

I checked with David Smith and he says it's been working for him, and we won't find the edge cases till more users hammer on it. ObjC-derived classes however still need some significant runtime interop work.

While we're here, make the 'unimplemented' error message less crashy by just skipping layout of non-fixed fields we can't lay out in ObjC classes yet.

Swift SVN r18797