[clangd] Decouple preambleworker from astworker, NFCI

Summary:
First step to enable deferred preamble builds. Not intending to land it
alone, will have follow-ups that will implement full deferred build
functionality and will land after all of them are ready.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76125