[clangd] Handling text editor/document lifetimes in vscode extension.

Summary:
Just reapplies highlightings for all files when visible text editors change. Could find the correct text editor that should be reapplied but going for a simple implementation instead.
Removes the associated highlighting entry from the Colorizer when a text document is closed.

Reviewers: hokein, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66735

llvm-svn: 369911