Correct several behaviors of availability checking in unavailable contexts that
were inconsistent with the checking model:
- Avoid diagnosing unintroduced and obsolted declarations in contexts that are
unavailable in the same domain.
- Diagnose unavailability normally in type signature contexts.
Protocol requirement witnesses cannot only be available in a custom
availability domain if the requirement does not have the same availability
constraint.
Resolves rdar://156462516.
Serialization and IRGen don't yet support opaque return types that would depend
on querying availability of a custom domain so we need to reject this code to
avoid mis-compiling it.
