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

[Aleksandr Platonov via cfe-commits]

https://github.com/ArcsinX approved this pull request.

Tested on CMake project again:
- new and old paths are the same: works good, as expected
- new and old paths are different: after moving the project ~20 files are reindexed by clangd, but I think this is acceptable for now and we can investigate this later.

Thanks for implementing index path mapping!

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