The reason why we are doing this is that this combination of read/set forces the
compiler to emit a copy if we want to emit a modify operation.
The reason why we are forced to emit such a copy is that:
1. _read provides a guaranteed value in memory
2. performing a modify requires an owned value in memory.
This together implies that the only way we can do this is to copy from the _read
into temporary memory. But we have a noncopyable type so we can't do this.
rdar://112915525
