Explore using spare bits in references for ARC optimizations

Ahead of ABI Stability, explore using spare bits in references to store local reference counts in order to perform some ARC operations more efficiently . These would need to be flushed to the object whenever a reference may escape or the local reference count reaches zero. If these local reference counts can cross ABI boundaries, then such a change will have to be implemented in an ABI-additive way with deployment target checking.