[clangd] Cleanup: make diagnostics callbacks from TUScheduler non-racy

Summary:
Previously, removeDoc followed by an addDoc to TUScheduler resulted in
racy diagnostic responses, i.e. the old dianostics could be delivered
to the client after the new ones by TUScheduler.

To workaround this, we tracked a version number in ClangdServer and
discarded stale diagnostics. After this commit, the TUScheduler will
stop delivering diagnostics for removed files and the workaround in
ClangdServer is not required anymore.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: javed.absar, ioeric, MaskRay, jkorous, arphaman, jfb, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D54829

llvm-svn: 347468