Introduce swift-xcodegen

This is a tool specifically designed to generate
Xcode projects for the Swift repo (as well as a
couple of adjacent repos such as LLVM and Clang).
It aims to provide a much more user-friendly experience
than the CMake Xcode generation (`build-script --xcode`).

utils/swift-xcodegen/Sources/SwiftXcodeGen/Concurrency/MutexBox.swift

@@ -0,0 +1,39 @@
+//===--- MutexBox.swift ---------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+final class MutexBox<T>: @unchecked Sendable {
+  private let lock = Lock()
+  private var value: T
+
+  init(_ value: T) {
+    self.value = value
+  }
+
+  @inline(__always)
+  func withLock<R>(_ body: (inout T) throws -> R) rethrows -> R {
+    try lock.withLock {
+      try body(&value)
+    }
+  }
+}
+
+extension MutexBox {
+  convenience init<U>() where T == U? {
+    self.init(nil)
+  }
+  convenience init<U, V>() where T == [U: V] {
+    self.init([:])
+  }
+  convenience init<U>() where T == [U] {
+    self.init([])
+  }
+}