Use type references for better error messages when relating to a union type

[DanielRosenwasser]

When relating A<Foo> to A<Bar> | B<Baz> | C<Kwah>, if the check fails, it is highly likely that the user wants to see the elaboration from A<Foo> to A<Bar>, rather than for A<Foo> to B<Baz> or A<Foo> to C<Kwah>.

We should try to find candidates when source types are type references, and target types are unions.

As a real-world example, here's an error message where TypeScript had the chance to relate ComponentClass<...> to ComponentClass<...> | StatelessComponent<...>:

https://news.ycombinator.com/item?id=17244638

This should also work for if two types have a well-known type alias.

Vaguely related to #6541?