Now that that's stabilized, we don't have to keep them in Swift 4 mode
any longer. (Arguably we don't need the CMake variable at all, but it
may be useful again in the future.)
rdar://problem/49040980
Magic symbols of the form $ld$install_name$os9.0$@rpath/libswiftCore.dylib tell the linker to use that install name when targeting that OS version. Use these symbols to specify an @rpath install name for all back-deployment libraries when targeting watchOS 2.0-5.1, iOS 7.0-12.1, and macOS 10.9-10.14.
rdar://problem/45027809
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.
