Nested excess property checking for discriminated unions

[sandersn]

Fixes #21187

Previously, unions and intersections would never do nested excess property checks. When the unions have a discriminant, nested checks are fine because only constituents matching the discriminant will be checked.

[sandersn]

Looks like the linter failed because I forgot to add parameter-name comments.

[weswigham]

Looks good. We should remember to check if this gives us any RWC changes.

[sandersn]

Nope, no RWC changes. There's only one outstanding change at the moment, but that's from Andy's improvement to type parameter error spans.