[clangd] Store source file hash in IndexFile{In,Out}

Summary:
Puts the digest of the source file that generated the index into
serialized index and stores them back on load, if exists.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D54693

llvm-svn: 347235