[clangd] Boost scores for decls from current file in completion

Summary: This should, arguably, give better ranking.

Reviewers: ioeric, sammccall

Reviewed By: sammccall

Subscribers: mgorny, klimek, MaskRay, jkorous, mgrang, cfe-commits

Differential Revision: https://reviews.llvm.org/D46943

llvm-svn: 333906