[SR-12154] Fix vtable entry type mismatch verification error for `@differentiable(wrt: self)`

[dan-zheng]

Previous ID	SR-12154
Radar	None
Original Reporter	@dan-zheng
Type	Sub-task

Additional Detail from JIRA

Votes	0
Component/s
Labels	Sub-task
Assignee	None
Priority	Medium

md5: 66daea3fab4a15a8539d2d766d092b41

is blocked by:

• TF-625 [AD] Change differentials/pullbacks to be maximally indirect

Issue Description:

Reproducer:

class Super : Differentiable {
  var base: Float = 0
  // Dummy to make `Super.AllDifferentiableVariables` be nontrivial.
  // var _nontrivial: [Float] = []

  @differentiable(wrt: self)
  func f() -> Float {
    return 1
  }
}

class Sub : Super {
  @differentiable(wrt: self)
  override func f() -> Float {
    return 1
  }
}

SIL verification failed: vtable entry for #Super.f!1.jvp.1.S must be ABI-compatible
  ABI incompatible return values
  @convention(method) (@guaranteed Super) -> (Float, @owned @callee_guaranteed (@guaranteed Super.AllDifferentiableVariables) -> Float)
  @convention(method) (@guaranteed Sub) -> (Float, @owned @callee_guaranteed (Super.AllDifferentiableVariables) -> Float)

This error occurs because class Super is non-trivial but struct Super.AllDifferentiableVariables is trivial.
Should be fixed by TF-625 by making differentials/pullbacks maximally indirect.

[jkshtj]

Still crashes on 05/24 toolchain.