Certain targets don't support the async calling convention, so we first
add the feature check to avoid breaking the codegen/runtime while doing
gradual rollout for different targets.
Recent Swift uses 2 as the is-Swift bit when running on newer versions, and 1 on older versions. Since it's difficult or impossible to know what we'll be running on at build time, make the selection at runtime.
* cmake: Propagate SWIFT_DARWIN_ENABLE_STABLE_ABI_BIT to overlay builds.
* runtime: Clear the correct bit in getROData()
* test/IRGen/objc_class_export.swift: Allow either is-Swift bit.
* test/stdlib/SwiftObjectNSObject.swift: Allow either name for SwiftObject.