[6.2][Runtime] Fix a false metadata cycle diagnostic when threads race to instantiate cyclical metadata.

[mikeash]

Cherry-pick #80505 to release/6.2.

The metadata creation system detects cycles where metadata depends on other metadata which depends on the first one again and raises a fatal error if the cycle can't be fulfilled.

Some cycles can be fulfilled. The cycle may involve a requirement for a metadata state less than full transitive completeness which can be reached without resolving the entire cycle. We only want to raise a fatal error when we detect a cycle that can't be fulfilled.

Normally this happens because the cycle checking in blockOnMetadataDependency only sees a cycle when it can't be fulfilled. Metadata initialization is advanced as far as it can be at each stage, so a cycle that can be fulfilled will see a fulfilling state and won't generate the dependency in the first place, since we only generate dependencies that haven't yet been met.

However, when two threads race to create types in a cycle, we can end up with such a dependency, because the dependency may be generated before another thread fulfilled yet. The cycle checker doesn't account for this and incorrectly raises a fatal error in that case.

Fix this by checking the cyclic dependency against the metadata's current state. If we have a dependency that's already been fulfilled, then there isn't really a dependency cycle. In that case, don't raise a fatal error.

rdar://135036243

[mikeash]

@swift-ci please test