[clangd] Build Dex index after loading all shards in BackgroundIndex.

Summary:
Currently after loadding all shards, we use MemIndex which has poor query
performance, we should use Dex.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59350

llvm-svn: 356126