Fix orelse type resolution

[Sekky61]

Fixes #1330

See the added tests, they make it pretty clear. One example:

const foo: ?i32 = 5;
const b<cursor>ar = foo orelse foo;

outputs

?i32

This is my first PR so I'd be happy for any pointers.

[Sekky61]

I have just noticed #1728, so I will work on that before opening the PR.

[Sekky61]

In the end I opted for a simpler solution. I noticed orelse has basically the same problem as if and others -- the unification of the types from both child nodes / branches.

I looked into resolvePeerTypes and how the Either type is used but that may be a discussion for another issue.