[clangd] Implement BOOST iterator

This patch introduces BOOST iterator - a substantial block for efficient
and high-quality symbol retrieval. The concept of boosting allows
performing computationally inexpensive scoring on the query side so that
the final (expensive) scoring can only be applied on the items with the
highest preliminary score while eliminating the need to score too many
items.

Reviewed by: ilya-biryukov

Differential Revision: https://reviews.llvm.org/D50970

llvm-svn: 340409