Fixes another hard to test problem with iterator invalidation.

As every match call can recursively call back into the memoized match
via a nested traversal matcher (for example:
stmt(hasAncestor(stmt(hasDescendant(stmt(hasDescendant(stmt()))))))),
and every memoization step might clear the cache, we must not store
iterators into the result cache when calling match on a submatcher.

llvm-svn: 186411