Breaking change 56466: Compute closures of type schemas in the upper bound computations in the CFE as specified

[Chloe Stefantsova]

The implementation of the algorithm for computing the upper bound of type schemas will be aligned with the specification and the implementation of it in the Analyzer.

This change is expected to have very low impact on real-world code. But in principle it could cause changes in runtime behavior.

For additional details please see https://github.com/dart-lang/sdk/issues/56466.

If this breaking change poses a severe problem, please leave comments in the linked issue.

[Chloe Stefantsova]

This change is being implemented.