[clangd] Expose Code Completion score to the client

Summary:
Make it possible for the client to adjust the ranking by using the score Clangd
calculates for the completion items.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74547