* Replace stdlib and test/stdlib 9999 availability.
macOS 9999 -> macOS 10.15
iOS 9999 -> iOS 13
tvOS 9999 -> tvOS 13
watchOS 9999 -> watchOS 6
* Restore the pre-10.15 version of public init?(_: NSRange, in: __shared String)
We need this to allow master to work on 10.14 systems (in particular, to allow PR testing to work correctly without disabling back-deployment tests).
The SDK directory is now confusing as the Windows target also has a SDK
overlay. In order to make this more uniform, move the SDK directory to
Darwin which covers the fact that this covers the XNU family of OSes.
The Windows directory contains the SDK overlay for the Windows target.