[clangd] Enable async preambles by default

Summary:
We've been testing this internally for a couple weeks now and it seems
to be stable enough. Let's flip the flag before branch cut to increase testing
coverage and have enough time to revert if need be.

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83290