[clangd] Allow consuming limited number of items

This patch modifies `consume` function to allow retrieval of limited
number of symbols. This is the "cheap" implementation of top-level
limiting iterator. In the future we would like to have a complete limit
iterator implementation to insert it into the query subtrees, but in the
meantime this version would be enough for a fully-functional
proof-of-concept Dex implementation.

Reviewers: ioeric, ilya-biryukov

Reviewed by: ioeric

Differential Revision: https://reviews.llvm.org/D50500

llvm-svn: 339426