This is still useful for the feature where deprecated declarations are
allowed to be used within a context that's marked deprecated. There's
probably a better balance to be found here, because marking the
extension deprecated still does not mark all the members deprecated,
but for now it still has an effect, and we shouldn't produce a warning
for legitimately making use of that effect.
The warning was added in f21d9f332, mostly as an opportunity "fix".
https://bugs.swift.org/browse/SR-7577
Previously, we treated this as an attempt to widen the availability
of a member beyond its context, but it's really a different axis of
availability.
Also, warn about using @available on extensions without an OS, which
we just completely ignore right now.
rdar://problem/32632327
