[ASTMatchers] Avoid recursion in ancestor matching to save stack space.

A recent change increased the stack size of memoizedMatchesAncestorOfRecursively
leading to stack overflows on real code involving large fold expressions.
It's not totally unreasonable to choke on very deep ASTs, but as common
infrastructure it's be nice if ASTMatchFinder is more robust.
(It already uses data recursion for the regular "downward" traversal.)

Differential Revision: https://reviews.llvm.org/D86964