[AutoDiff] Fix unsilenceable `@noDerivative` warning. (#28494)
Previously, `@noDerivative` warnings for `Differentiable` derived conformances
were unsilenceable for conditional `Differentiable` conformances: adding
`@noDerivative` in the nominal type declaration was rejected by `@noDerivative`
attribute type-checking.
Now, adding an explicit `@noDerivative` attribute silences these warnings.
`@noDerivative` can always be added in the nominal type declaration because
cross-file derived conformances are rejected.
Add `@noDerivative` tests for `Differentiable`-conforming structs/classes.
Resolves TF-1018.
