[clangd] Provide a way to publish highlightings in non-racy manner

Summary:
By exposing a callback that can guard code publishing results of
'onMainAST' callback in the same manner we guard diagnostics.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: javed.absar, MaskRay, jkorous, arphaman, kadircet, hokein, jvikstrom, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64985

llvm-svn: 366577