[clangd] Merge index-provided completions with those from Sema.
Summary:
- we match on USR, and do a field-by-field merge if both have results
- scoring is post-merge, with both sets of information available
(for now, sema priority is used if available, static score for index results)
- limit is applied to the complete result set (previously index ignored limit)
- CompletionItem is only produces for the returned results
- If the user doesn't type a scope, we send the global scope for completion
(we can improve this after D42073)
Reviewers: ioeric
Subscribers: klimek, ilya-biryukov, mgrang, cfe-commits
Differential Revision: https://reviews.llvm.org/D42181
llvm-svn: 322945