[clangd] Build ASTs only with fresh preambles or after building a new preamble

Summary:
This is another step for out-of-order preamble builds. To keep the
diagnostic behavior same, we only build ASTs either with "usable" preambles,
the ones that are fully applicable to a given ParseInput, or after building a
new preamble. Which is the same behaviour as what we do today. ASTs
built in the latter is called golden ASTs.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76725