[clangd] Run PreambleThread in async mode behind a flag

Summary: Depends on D80198.

This patch implies ASTs might be built with stale preambles without
blocking for a fresh one. It also drops any guarantees on every preamble
version being built. In case of multiple preamble build requests, in
addition to being debounced.

Any preamble requested with a WantDiags::Yes will always be built, this
is ensured by blocking enqueueing of any subsequent reqest.

AST worker will still block for initial preamble to reduce duplicate
work.

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80293