[clang-tools-extra] [clangd] Add background index path remapping tool (PR #185482)

[Justin Cady via cfe-commits]

justincady wrote:

> So:
> 
> * We renamed 20 files, but old and new paths were the same
> * Number of `*.idx` files reduced (we replaced some files)

Thank you for the review and for finding this issue. I'm investigating now, and I will report back.

https://github.com/llvm/llvm-project/pull/185482