[cland] Dex: fix/simplify short-trigram generation

Summary:
1) Instead of x$$ for a short-query trigram, just use x
2) Make rules more coherent: prefixes of length 1-2, and first char + next head
3) Fix Dex::fuzzyFind to mark results as incomplete, because
   short-trigram rules only yield a subset of results.

Reviewers: ioeric

Subscribers: ilya-biryukov, jkorous, mgrang, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52808

llvm-svn: 343775