Rollup merge of #150454 - tiif:move_variadic, r=oli-obk

cleanup: move c-variadic arguments handling into compute_inputs_and_output

Previously, ``unnormalized_input_tys`` needs to be mutable because the c_variadic arguments are added to ``unnormalized_input_tys`` outside of ``compute_inputs_and_output``. This could have been done together in ``compute_inputs_and_output``.