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

[Nico Weber via cfe-commits]

nico wrote:

As far as I can tell, the new binary does a bunch of string processing, but it depends on the full clangDaemon library, meaning building the new binary requires compiling all of clang. Could the new binary have fewer dependencies, to make it faster to build, and likely smaller too?

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