Prevent assert in ASTMatchFinder.

If nodes without memoization data (e.g. TypeLocs) are bound to specific
names, that effectively prevents memoization as those elements cannot be
compared effectively. If it is tried anyway, this can lead to an assert
as demonstrated in the new test.

In the long term, the better solution will be to enable DynTypedNodes
without memoization data. For now, simply skip memoization instead.