Allow `AsyncSequence.flatMap` to be defined with "incorrect" availability,
meaning that the function can refer to the `Failure` associated type
in its where clause even though the function is back-deployed to
before the `Failure` associated type was introduced.
We believe this is safe, and that this notion can be generalized to any
use of an associated type in a same-type constraint of a function
(yes, it sounds weird), but for now introduce a narrower hack to see
how things work in practice and whether it addresses all of the
source-compatibility concerns we've uncovered.
