This reproposes @lilyball’s fixes in https://github.com/apple/swift/pull/20103 while adding her fix to ensure observations are removed before observed objects in 32-bit testing.
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.