relates to:

• TF-1256 SIL: change "source" result index to result indices

Issue Description:

SILAutoDiffIndices is the data structure stored in SILDifferentiableAttr representing a "differentiable SIL function configuration": parameter indices and result index.

AutoDiffConfig is a new data structure used in SILDifferentiabilityWitness that stores parameter indices, result indices, and derivative generic signature.
AutoDiffConfig should replace all uses of SILAutoDiffIndices, e.g. in helper functions like SILFunctionType::getAutoDiffDerivativeFunctionType.