[DependencyScan] Use llvm::DefaultThreadPool instead of llvm::StdThreadPool

`llvm::StdThreadPool` is only available when LLVM_ENABLE_THREADS is enabled.
LLVM defines `DefaultThreadPool` type alias, which is always available
regardless of the threads enabled. This change allows building the library
without threads enabled, especially for Emscripten toolchain.

  ```
  #if LLVM_ENABLE_THREADS
  using DefaultThreadPool = StdThreadPool;
  #else
  using DefaultThreadPool = SingleThreadExecutor;
  #endif
  ```

Author: kateinoigakukun

include/swift/DependencyScan/ModuleDependencyScanner.h

@@ -180,7 +180,7 @@ class ModuleDependencyScanner {
   /// The available pool of workers for filesystem module search
   unsigned NumThreads;
   std::list<std::unique_ptr<ModuleDependencyScanningWorker>> Workers;
-  llvm::StdThreadPool ScanningThreadPool;
+  llvm::DefaultThreadPool ScanningThreadPool;
   /// Protect worker access.
   std::mutex WorkersLock;
   /// Count of filesystem queries performed