This is split out of #14029, to reduce the size of that PR, and to validate that this "fallback type" support in TypeInference doesn't come with a performance cost. It also improves the reliability and debuggability of our current (temporary) cycle handling.

In order to recover from a cycle, we have to be able to construct a "default" TypeInference where all expressions and definitions have some "default" type. In our current cycle handling, this "default" type is just unknown or a todo type. With fixpoint iteration, the "default" type will be Type::Never, which is the "bottom" type that fixpoint iteration starts from.

Since it would be costly (both in space and time) to actually enumerate all expressions and definitions in a scope, just to insert the same default type for all of them, instead we add an optional "missing type" fallback to TypeInference, which (if set) is the fallback type for any expression or definition which doesn't have an explicit type set.

With this change, cycles can no longer result in the dreaded "Missing key" errors looking up the type of some expression.