[clangd] Generate incomplete trigrams for the Dex index

This patch handles trigram generation "short" identifiers and queries.
Trigram generator produces incomplete trigrams for short names so that
the same query iterator API can be used to match symbols which don't
have enough symbols to form a trigram and correctly handle queries which
also are not sufficient for generating a full trigram.

Reviewed by: ioeric

Differential revision: https://reviews.llvm.org/D50517

llvm-svn: 339548