[clangd] Provide a way to publish highlightings in non-racy manner
By exposing a callback that can guard code publishing results of
'onMainAST' callback in the same manner we guard diagnostics.
Reviewed By: sammccall
Subscribers: javed.absar, MaskRay, jkorous, arphaman, kadircet, hokein, jvikstrom, cfe-commits
Differential Revision: https://reviews.llvm.org/D64985