[clangd] Build and test IndexBenchmark in check-clangd

Summary:
Include IndexBenchmark in check-clangd to make sure we won't forget to update
it when doing breaking changes; also fix an out-of-date test input.

Reviewers: ilya-biryukov

Subscribers: mgorny, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D54998

llvm-svn: 347753