[clangd] Implement proximity path boosting for Dex

This patch introduces `PathURI` Search Token kind and utilizes it to
uprank symbols which are defined in files with small distance to the
directory where the fuzzy find request is coming from (e.g. files user
is editing).

Reviewed By: ioeric

Reviewers: ioeric, sammccall

Differential Revision: https://reviews.llvm.org/D51481

llvm-svn: 341542