It's also not a backwards jump in do-while, unless it's in the loop's
condition, which is a stupid "feature" IMO. As you can probably tell
from the comments added in this commit.
This makes the `returns() implies` checker slightly cleaner, and also
fixes the case that I've missed where in RHS of `x ?:` type of `x` was
not set to `Nothing?`.
To do so, inside the root cause of inapplicable candidate errors,
we will record expected/actual type of receiver, if any.
That will help identifying inapplicable calls on nullable receiver.
Nikolay Lunyak