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SIL differentiability witnesses are a new top-level SIL construct mapping "original" SIL functions to derivative SIL functions. SIL differentiability witnesses have the following components: - "Original" `SILFunction`. - SIL linkage. - Differentiability parameter indices (`IndexSubset`). - Differentiability result indices (`IndexSubset`). - Derivative `GenericSignature` representing differentiability generic requirements (optional). - JVP derivative `SILFunction` (optional). - VJP derivative `SILFunction` (optional). - "Is serialized?" bit. This patch adds the `SILDifferentiabilityWitness` data structure, with documentation, parsing, and printing. Resolves TF-911. Todos: - TF-1136: upstream `SILDifferentiabilityWitness` serialization. - TF-1137: upstream `SILDifferentiabilityWitness` verification. - TF-1138: upstream `SILDifferentiabilityWitness` SILGen from `@differentiable` and `@derivative` attributes. - TF-20: robust mangling for `SILDifferentiabilityWitness` names.