[AutoDiff] Tweak `Differentiable` associated struct synthesis. (#22030)
Relax condition for synthesizing `AllDifferentiableVariables` struct and
letting `TangentVector` and `CotangentVector` alias it.
Previously, the condition was "all stored properties have
`Self = TangentVector = CotangentVector = AllDifferentiableVariables`".
This is overly restrictive and doesn't support the case where properties
have all associated types equal to one another but `Self != AllDifferentiables`.
This occurs when the stored property type:
- Has any `@noDerivative` properties.
- Doesn't itself conform to `AdditiveArithmetic`.
The precise correct condition is "all stored properties have
`TangentVector = CotangentVector = AllDifferentiableVariables`".
swift
9165198d - [AutoDiff] Tweak `Differentiable` associated struct synthesis. (#22030)
