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
This avoids us having to pattern match every source file which should
help speed up the CMake generation. A secondary optimization is
possible with CMake 3.14 which has the ability to remove the last
extension component without having to resort to regular expressions. It
also helps easily identify the GYB'ed sources.
Old Swift and new Swift runtimes and overlays need to coexist in the same process. This means there must not be any classes which have the same ObjC runtime name in old and new, because the ObjC runtime doesn't like name collisions.
When possible without breaking source compatibility, classes were renamed in Swift, which results in a different ObjC name.
Public classes were renamed only on the ObjC side using the @_objcRuntimeName attribute.
This is similar to the work done in pull request #19295. That only renamed @objc classes. This renames all of the others, since even pure Swift classes still get an ObjC name.
rdar://problem/46646438
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.
