[clangd] Log directory when a CDB is loaded

Summary: Fixes https://github.com/clangd/clangd/issues/268

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73628