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

[Justin Cady via cfe-commits]

justincady wrote:

> * We don't seem to support Windows paths. This is probably acceptable at this stage, but it needs to be explicitly written into the code as a FIXME. Right now, this can only be determined by testing.

Thanks. Updated to explicitly note this.

> * We use both llvm::errs() and clangd Logger (elog) for logging. I think the clangd logger (log(), vlog(), etc.) should be used everywhere.

Agreed. Fixed as suggested.

> * It's not entirely clear why we depend on zlib in the test. The index can be either packed or unpacked.

Thanks for catching this. Removed.

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