[clangd] Unify path canonicalizations in the codebase

Summary:
There were a few different places where we canonicalized paths, each
one had its own flavor. This patch tries to unify them all under one place.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D55818

llvm-svn: 349618