[clangd] Add priority to completion item sort text

Summary:
This patch adds the priority of a completion item to the sort text of the
returned LSP result.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D34137

llvm-svn: 305454