[AutoDiff upstream] Add PullbackEmitter.

`PullbackEmitter` is a visitor that emits pullback functions. It implements
reverse-mode automatic differentiation, along with `VJPEmitter`.

Pullback functions take derivatives with respect to outputs and return
derivatives with respect to inputs. Every active value/address in an original
function has a corresponding adjoint value/buffer in the pullback function.

Pullback functions consume pullback structs and predecessor enums constructed
by VJP functions.

Author: dan-zheng