[clangd] BackgroundIndex stores shards to the closest project

Summary:
Changes persistance logic to store shards at the directory of closest
CDB. Previously we were storing all shards to directory of the CDB that
triggered indexing, it had its downsides.

For example, if you had two TUs coming from a different CDB but depending on the
same header foo.h, we will store the foo.h only for the first CDB, and it would
be missing for the second and we would never persist it since it was actually
present in the memory and persisted before.

This patch still stores only a single copy of a shard, but makes the directory a
function of the file name. So that the shard place will be unique even with
multiple CDBs accessing the file. This directory is determined as the first
directory containing a CDB in the file's parent directories, if no such
directory exists we make use of the home directory.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64745

llvm-svn: 366467