[clangd] Send highlighting diff beyond the end of the file.

Summary: This would make the client life (tracking the changes) easier.

Reviewers: jvikstrom

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66541

llvm-svn: 369884