[clangd][NFC] Explode ReceivedPreamble into a CV

Summary:
Instead of a notification, we make use of a CV and store the boolean on
LatestPreamble by converting it into an optional.

Depends on D80293.

Reviewers: sammccall

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80784