[clangd] Bump index version number.

Summary:
Though we don't have new changes to the index format, we have changes to
symbol collector, e.g. collect marcos, spelled references. Bump the
version to force background-index to rebuild.

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74127