[clangd] Implement LIMIT iterator

This patch introduces LIMIT iterator, which is very important for
improving the quality of search query. LIMIT iterators can be applied on
top of BOOST iterators to prevent populating query request with a huge
number of low-quality symbols.

Reviewed by: sammccall

Differential Revision: https://reviews.llvm.org/D51029

llvm-svn: 340605