build: allow building libswift with unified builds
build: allow building libswift with unified builds

When building with unified builds, `CMAKE_SOURCE_DIR` is not the Swift
repository.  Use the locally defined `SWIFT_SOURCE_DIR` variable to
reference the root of the Swift repository.  Because libswift uses
`project` in the CMakeLists, we cannot use `PROJECT_SOURCE_DIR` here.

atrick/swift,atrick/swift,roambotics/swift,apple/swift,JGiola/swift,gregomni/swift,roambotics/swift,glessard/swift,apple/swift,glessard/swift,gregomni/swift,gregomni/swift,benlangmuir/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,glessard/swift,benlangmuir/swift,atrick/swift,JGiola/swift,ahoppen/swift,apple/swift,ahoppen/swift,gregomni/swift,ahoppen/swift,rudkx/swift,gregomni/swift,atrick/swift,apple/swift,gregomni/swift,JGiola/swift,apple/swift,JGiola/swift,JGiola/swift,benlangmuir/swift,benlangmuir/swift,JGiola/swift,roambotics/swift,roambotics/swift,glessard/swift,apple/swift,glessard/swift,ahoppen/swift,rudkx/swift,ahoppen/swift,rudkx/swift,ahoppen/swift,rudkx/swift,atrick/swift,atrick/swift,rudkx/swift,rudkx/swift,roambotics/swift,roambotics/swift