Normalize target type after null-stripping unions in relationship checks

[ahejlsberg]

Fixes #43152.

[DanielRosenwasser]

Why does it work with the other unions? Do we normalize in other parts of isRelatedTo?

[ahejlsberg]

@DanielRosenwasser We first normalize both the source and target types. We then do some tests for common patterns, one of which is for an object type on the source side and a union that includes undefined and/or null on the target side. When we see this pattern, we immediately remove undefined and null from the target. But we then need to re-normalize the remaining target, and that's what this PR adds.