[clangd] Deduplicate Refs on the fly.

Summary:
Currently, we only do deduplication when we flush final results. We may
have huge duplications (refs from headers) during the indexing period (running
clangd-indexer on Chromium).

With this change, clangd-indexer can index the whole chromium projects
(48 threads, 40 GB peak memory usage).

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59092

llvm-svn: 355676