Introduce Registry::getCompletions.

This returns a list of valid (and useful) completions for a context (a list
of outer matchers), ordered by decreasing relevance then alphabetically. It
will be used by the matcher parser to implement completion.

Differential Revision: http://llvm-reviews.chandlerc.com/D2210

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199950 91177308-0d34-0410-b5e6-96231b3b80d8