Implements memoization for ancestor matching.

This yields a log(#ast_nodes) worst-case improvement with matchers like
stmt(unless(hasAncestor(...))).

Also made the order of visitation for ancestor matches BFS, as the most
common use cases (for example finding the closest enclosing function
definition) rely on that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177081 91177308-0d34-0410-b5e6-96231b3b80d8