[clang-tools-extra] Improve loading speed of indexes used by clangd (PR #156185)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 requested changes to this pull request.

Apologies, I had this filed away as still needing review comments to be addressed. (Github does a poor job comparing to Phabricator of making it clear whether a patch is ready for review or awaiting changes...)

> I need to do some more code reading to better understand the `FileShardedIndex` call site and whether using `BuilderExpectUnique` is appropriate for it

Closing the loop on this: `FileShardedIndex` takes a single built `RefSlab` (which is thus already deduplicated) as input, and shards it into per-file shards. The shards therefore do not need additional deduplication, so using `BuilderExpectUnique` is fine here.

https://github.com/llvm/llvm-project/pull/156185