[clangd] Refactor background-index shard loading

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64712

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@366458 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed