[clangd] Build index on preamble changes instead of the AST changes
This is more efficient and avoids data races when reading files that
come from the preamble. The staleness can occur when reading a file
from disk that changed after the preamble was built. This can lead to
crashes, e.g. when parsing comments.
We do not to rely on symbols from the main file anyway, since any info
that those provide can always be taken from the AST.
Reviewers: ioeric, sammccall
Reviewed By: ioeric
Subscribers: malaperle, klimek, javed.absar, MaskRay, jkorous, cfe-commits
Differential Revision: https://reviews.llvm.org/D47272