[clangd] Remove one-segment-skipping from Dex trigrams.

Summary:
Currently queries like "ab" can match identifiers like a_yellow_bee.
The value of allowing this for exactly one segment but no more seems dubious.
It costs ~3% of overall ram (~9% of posting list ram) and some quality.

Reviewers: ilya-biryukov, ioeric

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

Differential Revision: https://reviews.llvm.org/D52885

llvm-svn: 343777