[clangd] Dex: add Corpus factory for iterators, rename, fold constant. NFC

Summary:
- Corpus avoids having to pass size to the true iterator, and (soon) any
  iterator that might optimize down to true.
- Shorten names of factory functions now they're scoped to the Corpus.
  intersect() and unionOf() rather than createAnd() or createOr() as this
  seems to read better to me, and fits with other short names. Opinion wanted!
- DEFAULT_BOOST_SCORE --> 1. This is a multiplier, don't obfuscate identity.
- Simplify variadic templates in Iterator.h

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D52711

llvm-svn: 343589