[AutoDiff] Emit SIL diff witnesses into original function's IRGenModule. (#28237)
The differentiation transform does not yet canonicalize differentiability
witnesses so that they always have JVP/VJP functions. TF-894 tracks this.
Until then, emit SIL differentiability witness into the IRGenModule for
the original function. The significance of this is unclear;
`IRGenerator::getGenModule` seems relevant only for multi-threaded
compilation.
Resolves TF-960: crash when `SILDifferentiabilityWitness::getVJP` is null.