[clangd] Don't mangle workdir-relevant driver path in compile commands

Summary:
We can't resolve this (if it's a symlink) without further refactoring, but the
current behaviour is just incorrect.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82011