Update to Swift Preview 4

Fixes from SP3 commits for relative path calculation

atpkg @ 6fdd57f7

-Subproject commit 79f5d1dc15017965216aaf5b905c2ca142dd7125
+Subproject commit 6fdd57f7daa08b9ab19fced0392aeeb3edc87d9d

attools/src/Shell.swift

@@ -72,7 +72,7 @@ final class Shell : Tool {
         environment["ATBUILD_BIN_PATH"] = binPath.description
 
         environment["PWD"]=String(validatingUTF8: realpath(task.importedPath.description,nil))!
-        print("pwd set to",environment["PWD"])
+        print("pwd set to",environment["PWD"]!)
         return environment
     }
     func run(task: Task) {

attools/src/Tools.swift

@@ -118,8 +118,15 @@ func anarchySystem(_ cmd: String, environment: [String: String]) {
     let args: [String] =  ["sh","-c",cmd]
     let argv = args.map{ $0.withCString(strdup) }
     let env: [UnsafeMutablePointer<CChar>?] = environment.map{ "\($0.0)=\($0.1)".withCString(strdup) }
+    
+    let directory = try! FS.getWorkingDirectory()
+    defer {try! FS.changeWorkingDirectory(path: directory)}
+    if let e = environment["PWD"] {
+        try! FS.changeWorkingDirectory(path: Path(environment["PWD"]!))
+    }
     let status = posix_spawn(&pid, "/bin/sh",nil,nil,argv + [nil],env + [nil])
 
+
     if status != 0 {
         fatalError("spawn error \(status)")
     }

tests/fixtures/xcode_toolchain/test-3.swift

 //this is an error on Swift 2
 //but "xcode-beta" currently ships swift 2.2
 //so if we compile successfully, we used Xcode-beta compiler.
-enum E : ErrorProtocol {}
\ No newline at end of file
+enum E : Error {}
\ No newline at end of file