Re-design the convenience interfaces on MatchFinder.

First, this implements a match() method on MatchFinder; this allows us
to get rid of the findAll implementation, as findAll is really a special
case of recursive matchers on match.

Instead of findAll, provide a convenience function match() that lets
users iterate easily over the results instead of needing to implement
callbacks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174172 91177308-0d34-0410-b5e6-96231b3b80d8