Use Tarjan's SCC alg. to find self-referential types (#3621)

Previously we computed the fixed point of the parent-child relation to identify
self-referential HeapTypes in the TypeBuilder canonicalizer. That algorithm was
O(|V|^3) in the worst case and took over five seconds to find the
self-referential HeapTypes in an example program with just 1134 HeapTypes,
probably due to high allocation traffic from the std::unordered_map and
std::unordered_sets used to implement the parent-child graph's adjacency list.

This PR replaces that algorithm with Tarjan's strongly connected component
algorithm, which runs in O(|V|+|E|) and finds the self-referential HeapTypes in
the mentioned example program in under 30 ms. All strongly connected components
of more than one element in the HeapType parent-child graph correspond to sets
of mutually recursive HeapTypes that are therefore self-referential. The only
other self-referential HeapTypes are those that are not mutually recursive with
any other HeapTypes, but these are trivial to find because they must be their
own direct children.

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