If a type conditionally conforms to BidirectionalCollection, suffix's (and the
others) use of `index` ends up dispatching through `Collection.index` seemingly
because it is a protocol requirement. The intended function is
BidirectionalCollection's overloaded `index` (which _isn't_ connected to a
protocol requirement), which is called for non-conditional conformances. As
such, this is a work-around to stop code crashing.
Noticed in SR-8022, rdar://problem/41216424.
