A recent change to use the IR value generator to produce default values for arbitrary schemas exposed a class of issues related to cyclic input values.

For example, consider this sdl:
```graphql
input A { b: B = {} }
input B { a: A = {} }
```

The spec considers this configuration invalid (see [InputObjectDefaultValueHasCycle](https://spec.graphql.org/draft/#sec-Input-Object-Default-Value-Has-Cycle)), because the coerced default value for `A.b`  is an infinitely large object `{a: {b: {a: ...}}}`.

The change introduced in #998226 allowed generating more edge-casey default values in a schema, including invalid types like what's described above.

To make things slightly worse, these invalid schemas were only detected at execution time via a StackOverflow in graphql-java. I've put up a PR for graphql-java to reject these kinds of invalid schemas during the validation phase, which we should get the next time we update:
graphql-java/graphql-java#4253

This PR updates the IR value generator to not generate values like the above. It uses strongly-connected component analysis to generate explicit values for fields where required. This allows us to keep generating interesting edge cases that we can use for testing.

Github-Change-Id: 999823
GitOrigin-RevId: 7e54ef25732b5bb8245d1a86cd828b6fc9d3edda