[Support] Implement llvm::Registry::iterator via llvm_iterator_facade

Summary:
Among other things, this allows using STL algorithms like 'find_if' over
llvm::Registry.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D56854

llvm-svn: 351566