[clangd] fixes semantic highlighting test

Summary: fixes https://github.com/clangd/clangd/issues/176

Patch by liu hui!

Reviewers: ilya-biryukov, hokein, sammccall

Reviewed By: hokein

Subscribers: MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D70078