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/Generator/ClangTargetSource.swift

@@ -0,0 +1,30 @@
+//===--- ClangTargetSource.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
+//
+//===----------------------------------------------------------------------===//
+
+/// The path at which to find source files for a particular target.
+struct ClangTargetSource {
+  var name: String
+  var path: RelativePath
+  var mayHaveUnbuildableFiles: Bool
+  var inferArgs: Bool
+
+  init(
+    at path: RelativePath, named name: String,
+    mayHaveUnbuildableFiles: Bool,
+    inferArgs: Bool
+  ) {
+    self.name = name
+    self.path = path
+    self.mayHaveUnbuildableFiles = mayHaveUnbuildableFiles
+    self.inferArgs = inferArgs
+  }
+}