These builds are meant to be as close to Xcode train builds as possible, so it
makes sense to treat them this way instead of letting them be treated as OS
builds or making a new kind of B&I build for them.
Playgrounds trains are identifiable via the `RC_PLAYGROUNDS` environment
variable being set to `YES`; if that isn't set, then it's not a Playgrounds
train.
This addresses <rdar://problem/85511438>.
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.
