[libTooling] Support TransformerResult<void> in consumer callbacks

Support `TransformerResult<void>` in the consumer callback, which
allows generic code to more naturally use the `Transformer` interface
(instead of needing to specialize on `void`).

This also delete the specialization that existed within `Transformer`
itself, instead replacing it with an `std::function` adapter.

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D122499