[clangd] Dex: FALSE iterator, peephole optimizations, fix AND bug

Summary:
The FALSE iterator will be used in a followup patch to fix a logic bug in Dex
(currently, tokens that don't have posting lists in the index are simply dropped
from the query, changing semantics).

It can usually be optimized away, so added the following opmitizations:
 - simplify booleans inside AND/OR
 - replace effectively-empty AND/OR with booleans
 - flatten nested AND/ORs

While working on this, found a bug in the AND iterator: its constructor sync()
assumes that ReachedEnd is set if applicable, but the constructor never sets it.
This crashes if a non-first iterator is nonempty.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D52789

llvm-svn: 343801