[AutoDiff] Fix derivative function protocol witness SILGen.
Fix SILGen verification failure for derivative function protocol witness thunks.
In `SILGenFunction::emitProtocolWitness`, `witnessSubs` (the witness
substitution map) does not have the same generic signature as `witnessFnRef`
(the result of `getWitnessFunctionRef`), which has additional differentiability
requirements.
Update `witnessSubs` to have the same generic signature as the witness callee.
Resolves SR-13229.
