[clangd] Store hash of command line in index shards.

Summary: This is to enable cache invalidation when command line flags changes.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64018

llvm-svn: 365121