Rewrite attr_availability_transitive_osx.swift to be more use a more thorough
cartesian product approach to testing possible combinations. Free up
Sema/availability.swift to run on platforms besides macOS.
NFC.
This makes diagnostics more verbose and accurate, because
it's possible to distinguish how many parameters there are
based on the message itself.
Also there are multiple diagnostic messages in a format of
`<descriptive-kind> <decl-name> ...` that get printed as
e.g. `subscript 'subscript'` if empty labels are omitted.
Muffle the diagnostic if the witness is in a different DeclContext than the
conformance.
Also, don't complain about the witness not being @objc if the requirement
is unavailable.
Finishes up <rdar://problem/26183366>.
This was easy with a bit of refactoring, but eventually I'd love to
converge override checking and witness checking logic a bit more.
Fixes <rdar://26183366>.
