[Evaluator] Enforce consistent results for cyclic requests #84745
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Record when we encounter a request cycle, and enforce that the outer step of the cycle also returns the default value. This fixes a couple of crashers where we were ending up with conflicting values depending on whether the request was queried from within the cycle or from outside it. This also allows us to flatten recursive ErrorTypes, which were previously load-bearing.
To help avoid additional downstream diagnostics for recursive generic signature computation, this PR also changes the placeholder generic signature logic to use same-type ErrorType requirements, and adds a checks to avoid downstream diagnostics in type resolution and conformance checking.
Resolves #57045