[clangd] Serialize onDiagnosticsReady callbacks for the same file.

Summary:
Calls to onDiagnosticsReady were done concurrently before. This sometimes
led to older versions of diagnostics being reported to the user after
the newer versions.

Reviewers: klimek, bkramer, krasimir

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38032

llvm-svn: 313754