Fix: Crash when comparing recursive models

[timotheeguerin]

fix #1991

Compiler would get into a infinite recursion when comparing recursive models.

model A {
  a: A;
}

model B {
  a: B;
}
model Test<T extends A> {}

alias T1 = Test<B>;

[github-actions]

Changes in this PR will be published to the following url to try(check status of TypeSpec Pull Request Try It pipeline for publish status):
Playground: https://cadlplayground.z22.web.core.windows.net/prs/2379/

Website: https://cadlwebsite.z1.web.core.windows.net/prs/2379/

[bterlson]

I feel like "unknown" describes this situation better than "maybe", maybe.

[timotheeguerin]

Typescript also has unknown so I kept the same naming

from their docs

 * Generally, Ternary.Maybe is used as the result of a relation that depends on itself, and
 * Ternary.Unknown is used as the result of a variance check that depends on itself. We make
 * a distinction because we don't want to cache circular variance check results.

[bterlson]

😂 I stand corrected