[clangd] Emit publishSemanticHighlighting in LSP if enabled

Summary: Emit publishSemanticHighlighting in LSP if enabled

Reviewers: hokein, kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63919

llvm-svn: 365111