[libcxx] Export directory_iterator in module build (llvm#129195)

This is workaround for the link error like https://crbug.com/390537876.

Might be related to llvm#120108
too.

Author: atetubou

libcxx/include/module.modulemap

@@ -1232,7 +1232,11 @@ module std [system] {
   module filesystem {
     module copy_options                   { header "__filesystem/copy_options.h" }
     module directory_entry                { header "__filesystem/directory_entry.h" }
-    module directory_iterator             { header "__filesystem/directory_iterator.h" }
+    module directory_iterator             {
+      header "__filesystem/directory_iterator.h"
+      // This is a workaround for https://github.com/llvm/llvm-project/issues/120108.
+      export *
+    }
     module directory_options              { header "__filesystem/directory_options.h" }
     module file_status                    { header "__filesystem/file_status.h" }
     module file_time_type                 { header "__filesystem/file_time_type.h" }