[SR-13464] Missing support for classes in forward-mode AD

[swift-ci]

Previous ID	SR-13464
Radar	None
Original Reporter	aefr (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 949a4cdb1f01d953b1483cc144f97c69

Issue Description:

Classes currently don't work with forward-mode AD, since some class-related SIL instructions (e.g. `ref_element_addr`) are not handled in `JVPCloner`. This causes crashes in some cases, e.g.

import _Differentiation

final class Class: Differentiable {
  var x: Float = 0

  struct TangentVector: Differentiable, AdditiveArithmetic {
    var x: Float
  }
  func move(along direction: TangentVector) {}
}

@differentiable
func testClass(_ c: Class) -> Float {
  var tmp = c
  return tmp.x // tmp.x is not visited
}