[AutoDiff] Support differentiation of switch_enum.

[dan-zheng]

Handle switch_enum terminator during VJP and adjoint generation.
Necessary step for differentiating for-in loops, which contain
optional iterator next() values.

Diagnose differentiation of active enum values, which requires
further adjoint generation support.

---

Example:

struct Dense : Differentiable {
  var w1: Float
  @noDerivative var w2: Float?

   @differentiable
  func callAsFunction(_ input: Float) -> Float {
    if let w2 = w2 {
      return input * w1 * w2
    }
    return input * w1
  }
}
expectEqual((Dense.AllDifferentiableVariables(w1: 10), 20),
            Dense(w1: 4, w2: 5).gradient(at: 2, in: { dense, x in dense(x) }))
expectEqual((Dense.AllDifferentiableVariables(w1: 2), 4),
            Dense(w1: 4, w2: nil).gradient(at: 2, in: { dense, x in dense(x) }))

[dan-zheng]

Diagnosing active enum values is necessary because adjoint generation doesn't propagate adjoint values of enum associated values correctly.

Support is non-trivial because switch_enum operand and successor block arguments have different types: the operand has an enum type but successor block arguments have associated values' type. Adjoint value propagation needs to construct enum adjoint value from associated values' adjoint values.

[dan-zheng]

It seems necessary to also handle switch_enum_addr, but I didn't find Swift functions containing switch_enum_addr that also don't contain active enum values. A quick searches shows that switch_enum_addr is generated during later SIL passes and also for optional force-unwrapping.

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

https://ci-external.swift.org/job/swift-PR-TensorFlow-Linux is down, ran tests locally.

$ ./swift/utils/build-script --preset tensorflow_test
...

Testing Time: 58.74s
  Expected Passes    : 10661
  Expected Failures  : 27
  Unsupported Tests  : 1385
-- check-swift-validation-linux-x86_64 finished --

Testing Time: 53.18s
  Expected Passes    : 1167
  Expected Failures  : 9
  Unsupported Tests  : 10897
-- check-swift-validation-optimize-linux-x86_64 finished --

Merging to unblock progress. Todo: add more tests.