[clangd] Support scope proximity in code completion.

Summary:
This should make all-scope completion more usable. Scope proximity for
indexes will be added in followup patch.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53131

llvm-svn: 344688