I had optimistically written the code here optimistically hoping #7837 would land in time for me to merge, but that didn't happen, so adjust some things to match the current 12-byte object header size on 32-bit, and introduce some ABI constants for the expected 32- and 64-bit object header sizes we can assert against so that we have some robustness when it eventually changes again. Implements rdar://problem/31768303.
A computed component needs:
- A stable identifier for equality checking. This can be either a static function reference for concrete property implementations, or a reference into a vtable or witness table for a dynamically dispatched property. This should correspond to a public ABI interface for public things.
- Getter and setter (TODO: and materializeForSet) references. These might be thunks to reach the necessary abstraction level for the keypath implementation, so they can't be used as stable identifiers directly.
- Separate out a uniquable KeyPathPattern that describes the context-free shape of the key path, with generic parameters and (eventually) subscript index slots factored out.
- Add component kinds for gettable and settable properties.
