[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