Handle AvoidMap recursions

[odersky]

Issue a diagnostic instead of crashing when there is a stack overflow

Avoids the crash in the minimized version of #15365

[odersky]

We still need to get to the bottom of why we get the cyclic lower bounds that lead to the problem.

[smarter]

We still need to get to the bottom of why we get the cyclic lower bounds that lead to the problem.

I have a simpler example where the constraint solver can create cycles like this: #15280, in this issue I proposed to detect the cycle at instantiation and emit an error then, but this isn't enough for operations like avoid that traverse the bounds of uninstantatiated type variables. So unless by some miracle we can come up with a cycle-free constraint solver, I guess we'd have to guard recursion in avoid and other operations by keeping track of seen type variables.

[dwijnand]

Btw, you can also just add // scalac: -Yno-deep-subtypes

[odersky]

OK, I am going to merge this for now, since a more fundamental fix is out of reach.