[clangd] Boost code completion results that are narrowly scoped (local, members)

Summary:
This signal is considered a relevance rather than a quality signal because it's
dependent on the query (the fact that it's completion, and implicitly the query
context).

This is part of the effort to reduce reliance on Sema priority, so we can have
consistent ranking between Index and Sema results.

Reviewers: ioeric

Subscribers: klimek, ilya-biryukov, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D47762

llvm-svn: 334026