[clang-tools-extra] Add --project-root to clangd (PR #155905)

[Mythreya Kuricheti via cfe-commits]

MythreyaK wrote:

For multi-workspace (not vscode workspace) projects, say a monorepo where each subproject is built individually (and therefore separate build dirs and compile_commands.json), is the idea that each subproject will (should?) now have its own clangd instance running? 

It mostly just worked in my experience so far, but wanted to understand this better. 

I am not completely sure how this might interact with vscode workspaces.  


https://github.com/llvm/llvm-project/pull/155905