[clangd] Cleans up the semantic highlighting resources if clangd stops.

Summary: Disposes of the vscode listeners when clangd crashes and reuses the old highlighter when it restarts. The reason for reusing the highlighter is because this way the highlightings will not disappear as we won't have to dispose of them.

Reviewers: hokein, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66743

llvm-svn: 370202