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

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 commented:

The patch updates look good to me, just one final nit: can we use the terminology `FallbackWorkingDirectory` consistently, i.e. in the name of the `CDBOpts` field, the name of the `applyWorkingDirectory` method, the various constructor parameters etc.?

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