[clangd] Use xxhash instead of SHA1 for background index file digests.

Summary:
Currently SHA1 is about 10% of our CPU, this patch reduces it to ~1%.

xxhash is a well-defined (stable) non-cryptographic hash optimized for
fast checksums (like crc32).
Collisions shouldn't be a problem, despite the reduced length:
 - for actual file content (used to invalidate bg index shards), there
   are only two versions that can collide (new shard and old shard).
 - for file paths in bg index shard filenames, we would need 2^32 files
   with the same filename to expect a collision. Imperfect hashing may
   reduce this a bit but it's well beyond what's plausible.

This will invalidate shards on disk (as usual; I bumped the version),
but this time the filenames are changing so the old files will stick
around :-( So this is more expensive than the usual bump, but would be
good to land before the v9 branch when everyone will start using bg index.

Reviewers: kadircet

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64306

llvm-svn: 365311