Rename *MatcherCreateCallback to *MatcherDescriptor, and its member run() to create().

The new names will be more appropriate when the objects are taught to return
type information for the code completer.

Differential Revision: http://llvm-reviews.chandlerc.com/D2208

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195539 91177308-0d34-0410-b5e6-96231b3b80d8