[clangd] Dex iterator printer shows query structure, not iterator state.

Summary:
This makes it suitable for logging (which immediately found a bug, to
be fixed in the next patch...)

Reviewers: ioeric

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52715

llvm-svn: 343580