[clangd] Make use of URIs in FileShardedIndex

Summary:
This makes FileShardedIndex more robust and gets rid of the need for a
URIToFileCache, as it is done by the callers now and it is only once per file,
rather than once per symbol.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79079