Make inert_cycle_breakers into a stack (#20231)

This is a straightforward fix for #20231 (closed). A bit annoying, but not hard, and quite local.

Side improvements: I made the collection of cycle breaker vars into more of its own distinct type. If you wanted, you could convince me to use a TyVarMap structure to make looking up more efficient, but I think looking up will a less common operation than just traversal.