[AA] Skip the layer of indirection in returning conservative results.

Historically, AA implementations chained to a following implementation
to answer recursive queries. This is no longer the case, but the legacy
lives on in a confusing phrasing of the return-a-conservative-value
paths. Let's just return "don't know" directly, where appropriate; the
current two-step way is confusing.

Differential Revision: https://reviews.llvm.org/D149100