Relates to test plan: dotnet#81074

First commit is a change that hopefully could be shared with "Closed Hierarchies" feature.

Related to dotnet/csharplang#9843

Related to dotnet#81074

Related to dotnet/csharplang#9902

…dotnet#82011)

…et#82325)

- Null pattern for classes
- TryGetValue nullable analysis

Supporting:
- ```union <name> ... ```

Align implementation with recent design changes:
- Case types are not nullable types (constructor parameter types are
unwrapped)
- Default nullability of the `Value` property is no longer inferred from
constructors. Annotations on the property declaration itself are used as
for any regular property.