[clangd] Bump index version and get rid of wrong assertion

Summary:
After rL360344, BackgroundIndex expects symbols with zero refcounts.
Therefore existing index files are no longer valid.

Assertion regarding finding target of a reference was wrong, since
background-index might still be going on or we might've loaded only some part of
the shards and might be missing the declaring shards for the symbol.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61734

llvm-svn: 360349