[clang-tools-extra] [clangd] Add background index path mapping (PR #180285)

[Aleksandr Platonov via cfe-commits]

ArcsinX wrote:

> Open Client A in VSCode with this clangd config:
> * `--log=verbose`
> * `--background-index=true`
> * `--background-index-path-mappings=${workspaceFolder}=/BUILDTOKEN`

This looks important, initially on ClientA there is `--background-index-path-mappings=${workspaceFolder}=/BUILDTOKEN` option. I tried and it works for me too, but does it work if you don't specify this option on Client A?
I.e.
- Client A has no `--background-index-path-mappings` option
- Client B has `--background-index-path-mappings=${workspaceFolder}=client_a_workspace_folder_here`

P.S. also it seems that modification of compile command doesn't lead to the related file reindexing. Thus, compilation command inside `*.idx` file looks unused. Maybe it's a clangd bug, this needs deeper investigation, but this can be important for path mappings. If this is a bug and it will be fixed, then compile commands paths inside *.idx files also must be mapped

https://github.com/llvm/llvm-project/pull/180285