GADTs: widen only top-level variant params

[abgruszecki]

Fixes #11565.

The change to an unrelated test file is to clarify what should and shouldn't be inferred.

[abgruszecki]

The fix is quite simple: we should only widen top-level variant type params - remember that the widening is there only to trick TypeComparer into doing comparison between the appropriate type params. The doc comment for the function even implies that only top-level params should be widened.

[smarter]

The fix is quite simple: we should only widen top-level variant type params - remember that the widening is there only to trick TypeComparer into doing comparison between the appropriate type params. The doc comment for the function even implies that only top-level params should be widened.

That whole comment should go in the commit message.

[abgruszecki]

Oh, I was going to add the comment to the commit message, but I got distracted. Anyway, it's the source already (it's one of those Huge Alex Comments), so I guess it's fine.