[clangd] Don't send semanticHighlights to clients that support semanticTokens.

Summary: This allows the standard mechanism to gracefully displace the old one.

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77206