Parent-Task:

• SR-13659 Retroactive derivative registration

Issue Description:

The differentiability_witness_function instruction is similar to class_method
and witness_method instructions.

It looks up a differentiability witness function (JVP, VJP, or transpose) for a
referenced function using SIL differentiability witnesses.

%vjp = differentiability_witness_function [vjp] [parameters 0 1] [results 0] \
    <T where T: Differentiable, T == T.TangentVector> \
    @generic : $@convention(thin) <T> (@in_guaranteed T, Float) -> @out T

differentiability_witness_function will be generated by the differentiation transform
and will be IRGen'd to extract witness functions from SIL differentiability witnesses.