Update for SE-0110 -- @NoReturn attribute replaced with Never type

Author: slavapestov

Sources/Commands/Error.swift

@@ -50,7 +50,7 @@ extension Error: FixableError {
     }
 }
 
-@noreturn public func handle(error: Any, usage: ((String) -> Void) -> Void) {
+public func handle(error: Any, usage: ((String) -> Void) -> Void) -> Never {
 
     switch error {
     case OptionParserError.multipleModesSpecified(let modes):

Sources/POSIX/exit.swift

@@ -10,6 +10,6 @@
 
 import libc
 
-@noreturn public func exit(_ status: Int32) {
+public func exit(_ status: Int32) -> Never {
     libc.exit(status)
 }

Sources/POSIX/getcwd.swift

@@ -28,7 +28,7 @@ import var libc.errno
 */
 public func getcwd() -> String {
 
-    @noreturn func error() {
+    func error() -> Never {
         fputs("error: no current directory\n", libc.stderr)
         libc.exit(2)
     }

Sources/Utility/Git.swift

@@ -99,7 +99,7 @@ public class Git {
         return Int(String(first))
     }
 
-    @noreturn public class func checkGitVersion(_ error: Swift.Error) throws {
+    public class func checkGitVersion(_ error: Swift.Error) throws -> Never {
         // Git 2.0 or higher is required
         if let majorVersion = Git.majorVersionNumber, majorVersion < 2 {
             // FIXME: This does not belong here.