Merge 'Unions' into main by AlekseyTs · Pull Request #82640 · dotnet/roslyn

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

- 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.

…n from

dotnet/roslyn#82640

Adding Custom roslyn compiler version via csp files

Adding performance benchmarks for difference between functional union, new version using native union, and just simple switch using native union

…n from

dotnet/roslyn#82640

Adding Custom roslyn compiler version via csp files

Adding performance benchmarks for difference between functional union, new version using native union, and just simple switch using native union