[clangd] Optionally use dex for the preamble parts of the dynamic index.

Summary:
Reuse the old -use-dex-index experiment flag for this.

To avoid breaking the tests, make Dex deduplicate symbols, addressing an old FIXME.

Reviewers: hokein

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

Differential Revision: https://reviews.llvm.org/D53288

llvm-svn: 344594