[clangd] Add xref for macros to FileIndex.

Summary:
Adds macro references to the dynamic index.
Tests added.
Also exposed a new API to convert path to URI in URI.h

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71406