Multithread processing source files

Previously every source file was formatted / linted one by one. On our
codebase this took a full project format from ~17 minutes to ~5 minutes.

Author: keith

Sources/swift-format/main.swift

@@ -69,18 +69,36 @@ fileprivate func main(_ arguments: [String]) -> Int32 {
 ///   - transform: A closure that performs a transformation on a specific source file.
 private func processSources(
   from paths: [String], configurationPath: String?,
-  transform: (FileHandle, String, Configuration) -> Int
+  transform: @escaping (FileHandle, String, Configuration) -> Int
 ) -> Int32 {
   var result = 0
+  let concurrentQueue = DispatchQueue(label: "com.queue.Concurrent", qos: .userInitiated, attributes: .concurrent)
+  let group = DispatchGroup()
+  let lock = NSLock()
   for path in FileIterator(paths: paths) {
-    guard let sourceFile = FileHandle(forReadingAtPath: path) else {
-      stderrStream.write("Unable to create a file handle for source from \(path).\n")
-      stderrStream.flush()
-      return 1
+    group.enter()
+    concurrentQueue.async {
+      defer { group.leave() }
+      guard let sourceFile = FileHandle(forReadingAtPath: path) else {
+        stderrStream.write("Unable to create a file handle for source from \(path).\n")
+        stderrStream.flush()
+        lock.lock()
+        result |= 1
+        lock.unlock()
+        return
+      }
+
+      let configuration = loadConfiguration(forSwiftFile: path, configFilePath: configurationPath)
+      let transformResult = transform(sourceFile, path, configuration)
+      if transformResult > 0 {
+        lock.lock()
+        result |= transformResult
+        lock.unlock()
+      }
     }
-    let configuration = loadConfiguration(forSwiftFile: path, configFilePath: configurationPath)
-    result |= transform(sourceFile, path, configuration)
   }
+
+  group.wait()
   return Int32(result)
 }