Revert "Revert r366458, r366467 and r366468"

This reverts commit 9c377105.

[clangd][BackgroundIndexLoader] Directly store DependentTU while loading shard

Summary:
We were deferring the population of DependentTU field in LoadedShard
until BackgroundIndexLoader was consumed. This actually triggers a use after
free since the shards FileToTU was pointing at could've been moved while
consuming the Loader.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64980

llvm-svn: 366559