[clangd][vscode] Turn on the semantic highlighting by default.

Summary:
We have turned on the flag manually and used it for a while, and don't see any
major issues, let's enable it by default.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68077

llvm-svn: 372969