Re-design the convenience interfaces on MatchFinder.

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.

llvm-svn: 174172