[clang-tools-extra] [clangd] Add background index format support to clangd-indexer (PR #175209)

[Nathan Ridge via cfe-commits]

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

Thanks for the update, and sorry for the long review turnaround time.

I think what the patch is doing now makes sense on a high-level, and we just need to sort out some of the details -- see comments inline.

https://github.com/llvm/llvm-project/pull/175209