[clangd] Simplify Dex query tree logic and fix missing-posting-list bug

Summary:
The bug being fixed: when a posting list doesn't exist in the index, it
was previously just dropped from the query rather than being treated as
empty. Now that we have the FALSE iterator, we can use it instead.

The query tree logic previously had a bunch of special cases to detect whether
subtrees are empty. Now we just naively build the whole tree, and rely
on the query optimizations to drop the trivial parts.

Finally, there was a bug in trigram generation: the empty query would
generate a single trigram "$$$" instead of no trigrams.
This had no effect (there was no posting list, so the other bug
cancelled it out). But we now have to fix this bug too.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52796

llvm-svn: 343802