The above referenced issue was causing a compiler crash when writing
differentiable protocols and corresponding implementers, without importing
the `_Differentiation` module.
Changes in this CR fix the issue by marking any `@differentiable`
attributes as invalid, if the `_Differentiation` module has not been
imported. This ignores the `@differentiable` attributes when the
protocol witnesses are being verified. Witness verification was previously
leading to an error (due to missing `@differentiable` attribute on the protocol
requirement implementer), and the corresponding diagnostic emission code was
then leading to a crash, because it was expecting the `_Differentiation`
module to be present.
