[clangd] Support returning a limited number of completion results.

Summary:
All results are scored, we only process CodeCompletionStrings for the winners.
We now return CompletionList rather than CompletionItem[] (both are valid).
sortText is now based on CodeCompletionResult::orderedName (mostly the same).

This is the first clangd-only completion option, so plumbing changed.
It requires a small clangd patch (exposing CodeCompletionResult::orderedName).

(This can't usefully be enabled yet: we don't support server-side filtering)

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D39852

llvm-svn: 318287