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

[Justin Cady via cfe-commits]

justincady wrote:

> 1. I checked on `CMake` project and it seems in some cases with the same filenames we can replace another `*.idx` file. I failed to reproduce this on a simple project, but it stable to reproduce on `CMake`:

Fixed (and explained) in 3b95b16f819fd1580af3433ab52072297e75f89e. Thanks again for noticing this! It would show up with any project with multiple files with the same basename.

> 2. Maybe we need an ability to control logs verbosity with command line option (like in clangd)

Sure! Added as suggested.

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