message: [Reproducers] Always pass the VFS to Swift
  Fix two places in SwiftASTContext where we need to pass LLDB's virtual
  file system to Swift.
  - The DWARFImporterDelegate wasn't setting the VFS.
  - The source manager no longer requires an OverlayFS, so we can pass
  the VFS directly.
diff: @@ -2670,10 +2670,8 @@ swift::CompilerInvocation &SwiftASTContext::GetCompilerInvocation() {
  swift::SourceManager &SwiftASTContext::GetSourceManager() {
  if (!m_source_manager_up) {
  - auto OverlayFS = llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem>(
  - new llvm::vfs::OverlayFileSystem(llvm::vfs::getRealFileSystem()));
  - OverlayFS->pushOverlay(FileSystem::Instance().GetVirtualFileSystem());
  - m_source_manager_up = std::make_unique<swift::SourceManager>(OverlayFS);
  + m_source_manager_up = std::make_unique<swift::SourceManager>(
  + FileSystem::Instance().GetVirtualFileSystem());
  }
  return *m_source_manager_up;
  }
  @@ -3319,7 +3317,8 @@ public:
  }
  clang::FileSystemOptions file_system_options;
  - clang::FileManager file_manager(file_system_options);
  + clang::FileManager file_manager(
  + file_system_options, FileSystem::Instance().GetVirtualFileSystem());
  clang_types.ForEach([&](lldb::TypeSP &clang_type_sp) {
  if (!clang_type_sp)
  return true;
  