Optional's `init_enum_data_addr` and `inject_enum_addr` instructions are generated in presence of non-loadable Optional values. The compiler used to treat these instructions as inactive, and this resulted in silent run-time
issues described in #64223.
The patch marks `init_enum_data_addr` as "active" if its Optional operand is also active, and in PullbackCloner we differentiate through it and the related `inject_enum_addr`.
However, we only determine this relation in simple cases when both instructions are in the same block. There is no def-use relation between them (both take the same Optional operand), so if there is more than one set of instructions
operating on the same Optional, or there is some control flow, we currently bail out.
In PullbackCloner, we walk over instructions in reverse order and start from `inject_enum_addr` and its `Optional<Wrapped>.TangentVector` operand. Assuming that is is already initialized, we emit an `unchecked_take_enum_data_addr` and set it as the adjoint buffer of `init_enum_data_addr`. The Optional value is
invalidated, and we have to destroy the enum data address later when we reach `init_enum_data_addr`.
Rename `move(along:)` to `move(by:)` based on the proposal feedback. The main argument for the change is that tangent vectors specify both a direction and a magnitude, whereas `along:` does not indicate that `self` is being moved by the specified magnitude.
Make `Optional` conditionally conform to `Differentiable` when the `Wrapped` type does.
`Optional.TangentVector` is a wrapper around `Wrapped.TangentVector?`.
Also, fix `Array.TangentVector.zeroTangentVectorInitializer`.
Resolves TF-1301.
