Split registry matcher resolution into a lookup phase and a construction phase.

The looked-up matchers will be used during code completion.

Differential Revision: http://llvm-reviews.chandlerc.com/D2207

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195534 91177308-0d34-0410-b5e6-96231b3b80d8