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/Error.swift

@@ -0,0 +1,30 @@
+//===--- Error.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
+//
+//===----------------------------------------------------------------------===//
+
+enum XcodeGenError: Error, CustomStringConvertible {
+  case pathNotFound(AbsolutePath)
+  case noSwiftBuildDir(AbsolutePath, couldBeParent: Bool)
+  case expectedParent(AbsolutePath)
+
+  var description: String {
+    switch self {
+    case .pathNotFound(let basePath):
+      return "'\(basePath)' not found"
+    case .noSwiftBuildDir(let basePath, let couldBeParent):
+      let base = "no swift build directory found in '\(basePath)'"
+      let note = "; did you mean to pass the path of the parent?"
+      return couldBeParent ? "\(base)\(note)" : base
+    case .expectedParent(let basePath):
+      return "expected '\(basePath)' to have parent directory"
+    }
+  }
+}