Properly reduce intersections of string literal and template literal types

[ahejlsberg]

Fixes #41161.

[weswigham]

Hm, this is fine, but the reduce operation has the potential to be very expensive on large input sets. Are we sure there's not a way re can rejigger how we read the cache to keep the operation cached (even when the final result is never)?

[ahejlsberg]

@weswigham I think it's going to be quite rare to have large intersection input sets that contain both string literal types and template literal types so I'm not too worried about it. I think the common pattern here will be intersecting a large union of string literal types with a template literal type, but that'll normalize to a large union of little intersections.