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

[Jason Williams via cfe-commits]

jasonwilliams wrote:

Hi both, I sadly ran out of time to work on this so will need help, I’m glad to have someone takeover if you have capacity. If not I’ll need to find some time to come back to it, I’m always happy to test if someone else picks it up 

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