[clangd] Use workspace root path as hint path for resolving URIs in workspace/symbol

Summary:
Some URI schemes require a hint path to be provided, and workspace root
path seems to be a good fit.

Reviewers: sammccall, malaperle

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D48290

llvm-svn: 335035